The only logical solution, really
"CAN'S BUSTED!" yelled Rainbow Dash, from all the way down the hall.
The other three ponies who were spending the night at Applejack's farmhouse - Fluttershy, Rarity, and Applejack herself - made their way to the bathroom, where Rainbow Dash stood, pointing an accusatory hoof at the toilet.
"There's no need to yell," said Rarity mildly.
"You guys were at the far end of the hall," said Rainbow Dash, cocking her head. "If I hadn't yelled, you wouldn't have heard me."
Applejack huffed. "I think what Rarity means is, y'all could have come back to the livin' room and told us in a normal tone of voice."
"I guess so," said Dash, who still didn't seem to understand why yelling had been less appropriate. "Anyway, your can's broken, and it's not my fault. I didn't even use it yet, I just saw that there was no water in there and I tried flushing it."
"Lemme take a look," said Applejack, gesturing for Dash to step aside. She lifted the lid off the tank, fiddled with the internal mechanisms, and tried the handle. Nothing. Applejack sighed. "Yeah, this ain't fixable without a replacement part, and the stores are all closed. And the humble Apple Family home ain't got but the one bathroom, I'm -" Applejack stopped and shook her head slightly, with a confused expression on her face.
"What's wrong, darling?" said Rarity, stepping closer to Applejack.
"Oh, nuthin'," said the farmpony. "Just... what's that Prench expression? Deja vu? Just got hit real hard with that." The other ponies all looked at each other, unsure what to make of this. "Anyhow," Applejack continued, "This is the only toilet on the premises, girls."
"Hrm," muttered Rarity. "I was rather hoping to make use of the facilities, er, soonish. Not to be crass, but the wonderful meal you served us was... on the large side, and..."
"I know how you feel," said Dash. "I gotta take a wicked dump too."
"Just so," said Rarity, sighing.
"Yeah, me too," said Applejack, and Fluttershy nodded. "So we got no workin' toilets, and all of us need to go. What's the most logical solution here?"
There was a moment of silence as the ponies considered their options. In the end, it was Fluttershy who spoke. "Um," she said, shifting her wings nervously, "We could... pee and poop in each others' mouths."
Another silence followed.
And then all at once, Rarity, Rainbow Dash and Applejack all nodded in agreement, all talking over each other as they complimented Fluttershy on the elegant, logical solution she'd proposed. Their words of affirmation died down, and then Applejack started talking logistics. "So we may as well go two-an'-two, right? I'll take my turn bein' a toilet for the first round, and I ain't particular about who uses me. Who else wants to go first?"
"Oh, uh, I'll go," said Fluttershy. "Whichever of you two wants to use me as a toilet is fine," she said, looking at Rarity and Rainbow Dash.
"Then it's decided," said Rarity. "Dashie, do you have a preference of whose mouth you'd wish to... use?"
Rainbow Dash laughed. "Never really thought about it. Applejack, I guess?"
The first two toilets - Fluttershy and Applejack - sat back on their haunches and angled their faces upward. "Go ahead and poop in my mouth," said Fluttershy. "Um, whenever you're ready."
"Oh, I'm more than ready, darling," said Rarity, lifting her tail and carefully backing her rear up to Fluttershy's face. "I have to serve you a drink before your main course, though." Her body relaxed, sinking into Fluttershy's face, as she released a stream of piss into the yellow pegasus' open mouth.
"Heh, I better get started too," said Dash, taking her position over Applejack. "I'm a racehorse and I have to piss exactly as much as you'd expect." Her stream was forceful, splattering off of Applejack's tongue and splashing back onto Dash's rear. She kept pissing long after Rarity had finished, forcing Applejack to gulp down mouthful after mouthful of piss to keep up with Dash's output.
As Dash was finishing up, Rarity was already repositioning herself so that her anus was directly over Fluttershy's open mouth. "Mmm, I hope you're ready," she said, and her body tensed as her asshole opened. She moaned with pleasure as the first thick piece fell and landed on Fluttershy's tongue. After the first piece, the rest came out in a rush, a torrent of brown turds that filled Fluttershy's mouth to overflowing. Fluttershy muttered meekly, but it was too soft and too muffled by shit for Rarity to hear, and she just kept going, piling shit onto shit as Fluttershy struggled to swallow it all.
Applejack's job was easier; Dash had been full of piss, but had comparatively little shit to deposit in the farmpony's mouth. The flow of shit from Dash was slower, allowing Applejack to swallow each turd one by one, moaning appreciatively as she did. Finally, Rainbow Dash lifted her ass off of Applejack and stepped away.
"Aw," said Applejack, "That all you got Dash? I'm good for more if you got any left in the tank!" She looked to the side, where Fluttershy was still stuggling to gulp down the heaping mouthful of shit that Rarity had deposited. "Aw, Rarity, I wish you'd used me! You gave Fluttershy a real generous portion, there!"
"Yes, well, Element of Generosity and all that," said Rarity. "Now then, Applejack, would you prefer to use me or Dash? You may as well get started." She plopped her flank down on the bathroom floor, and Rainbow Dash did the same.
"Hm, tough choice," said Applejack. "I s'pose I'll use you, Rarity." She backed up to Rarity's face, and Rarity widened her mouth, forming a seal around Applejack's backside with her lips. Applejack, confident that Rarity had full coverage, began to piss and shit simultaneously, giving Rarity a bit more than she bargained for.
"I know I already served you a big meal tonight, Rares," said the farmpony, chuckling, "but now I've got a lot of piss an' shit to fill yer belly with!" Rarity murmured as she swallowed, trying to keep an agreeable tone despite the considerable - and unexpected - level of effort that was required to keep up with Applejack's dump.
Meanwhile, Fluttershy swallowed the last of Rarity's load of poop. She gasped as her airways were freed up, and spent a moment just breathing. "Lemme know when you're ready, Fluttershy," said Rainbow Dash, still seated in the pony-toilet position. "I'm all set to eat your poop."
Fluttershy picked herself up off the floor and planted her butt on Rainbow Dash's face. "Um... here it comes," she said, and let out a high-pitched sound of effort as she began to push her load out of her ass. Dash felt a short burst of piss hit her tongue, followed by three smallish turds - and then it stopped. Dash swallowed and waited for more, but nothing came. Cautiously, she pulled her head back from Fluttershy's ass.
"That all you got?" Rainbow Dash asked, in a challenging tone.
"Um... sorry," said Fluttershy, "I guess I didn't have to go as badly as you other girls."
"Nah, that's okay!" said Dash. "I'd just psyched myself up for more, you know? I kinda wanted to prove I was the best toilet around!"
"Ain't that just like you," chuckled Applejack, whose own dump was just tapering off. She grunted as she pushed the last turd into Rarity's mouth, then turned around to look at her handiwork. "I think the title of Best Toilet might have to go to Rares, though - or at least the Most Hardwokin' Toilet!"
Rarity was indeed working hard, her eyes watering slightly as she gulped down Applejack's shit bit by bit. Piss had overflowed her mouth, spilling over her lips, but she'd caught all the poop and was slowly working her way through it, her throat moving the load down to her stomach bit by bit. Soon, her mouth cleared and she downed the final swallow, gasping for air even more desperately than Fluttershy had.
"My goodness," gasped Rarity, "That was quite an experience. I suppose I deserve it, though, for being so rough on Fluttershy."
"Oh, don't worry," said Fluttershy, "I had fun."
"I think we all did, Fluttershy," said Applejack. "Thanks fer suggestin' this."
"Yeah," said Rainbow Dash. "But... we're not gonna do this again when we poop out each others' poop, right? I don't think I'd be up for that."
"What?" said Applejack, recoiling slightly. "No, we ain't doin' that! Eatin' each others' double-poop? That'd just be weird." The other ponies nodded in agreement, and after taking a moment to brush their teeth, they all returned to Applejack's bedroom.