Depending on your bed, you might be able to use a large vinyl tablecloth (with flocking on the back. You could turn it upside down so they hop on that side for traction. Or put an old sheet on top of it. You’d probably have to keep replacing that obviously..
It could get pretty warm for you under the covers though! Hopefully, if they are allowed more frequent access to the bed, they will start to stop marking after a while?
Another idea is to pop a small litterbox on end of bed or a fabric incontinence chair/bed mat with a little hay on it?
I do understand the lure of letting them up on the bed with you. My single bun does. He is good about litter habits but when he is unwell, he tends to pee on the bed then.
Does the bedroom have more slippery flooring? Could that be part of why they like to binky up on the bed so much?
ETA: Out of curiosity, I just googled “waterproof doona cover”. They exist!