My rabbits stopped using their litter boxes...and it's been a year.

[akat]

A year ago I got a friend for my rabbit. They're both female, spayed, around 3 years old, and healthy. Both were meticulous about using their litter boxes before I began bonding them, but the second they were sharing their space, they started peeing and pooping everywhere, presumably for territorial reasons.
They have free roam of my bedroom, sitting room, and the hallway in between, and have four litter boxes in that area. The entire house is carpeted, and the entire carpet is destroyed. Multiple times a week I have to spend hours scrubbing urine out of the floors with a carpet-shampooing machine, then using vinegar to try and neutralize the smell. Every time, without fail, they soak it in pee by the next day.
I've closed them into a play pen before for weeks at a time, gradually expanding their space until they're using the litter boxes again, but once they have their normal space back, they revert back immediately. I don't know if it's the carpet that's the problem, or that they just don't like sharing litter boxes, but I don't know the solution for that as I can't remove the carpet in my rented home, and I don't know if I can force them to use separate litter boxes, even though they have plenty to choose from.
I'm beyond frustrated and feel like I've tried everything. Everyone told me to give it time, but it's been an entire year and it's seeming to only get worse. I'm feeling so hopeless and am in desperate need of help.

 

[FuzzyBunny ph]

how big is the litterbox? maybe they find it too small?

have your ruled out UTI? bunnies that are good with litterboxes then suddenly starts to pee outside the litterbox and in random places is said to be one of the signs of UTI

 

[Blue eyes]

I've closed them into a play pen before for weeks at a time, gradually expanding their space until they're using the litter boxes again, but once they have their normal space back, they revert back immediately.
I'm beyond frustrated and feel like I've tried everything. Everyone told me to give it time, but it's been an entire year and it's seeming to only get worse. I'm feeling so hopeless and am in desperate need of help.

It sounds like you were definitely making progress. They were using their boxes up to a point. That is the key point. It was after they 'have their normal space' back that it went downhill. There's your key. It means that the problem can be narrowed down to that window. What exactly were the conditions just before they got their normal space back? Either they need to spend a longer time at that level (where they were still going well) and/or they need an additional step or steps (gradual increments) between those last two steps.

It's possible that the jump from when they were using their boxes well to being given their normal space was just too big of a jump. When moving from one size space to a larger space, if one urine mistake is made then back they go to the prior size space. They should never be allowed to make more than one 'oops' in a newly enlarged area. One oops and back they go.

As for the carpeted area that they've already peed on... that odor lingers -- maybe not to our noses, but they can smell it. You may not be able to replace it in a rental, but you can cover it. You could try either cheap, low pile area rugs or even rolled linoleum laid directly on top (guard the edges). By covering the existing carpet and taking that more gradual approach, you may just be able to get over that hump.