I don’t know exactly why some bunnies choose to mark on their beds while others don’t. Both Jack and Vivian are good about not weeing on their beds, but a bunny I used to have, Bailey, would absolutely purposefully pee on her bed. She was also extremely territorial though, and so for her I knew she would do it to mark it.
Do you know for sure that BOTH do and it’s not just one that pees there? If they both do, they may be battling out who is top bun by peeing over each other’s smell.
Do you have any other animals in the house? The reason I ask is sometimes other animal scents can make bunnies want to reinforcing marking their area.
The other thing that could be causing it is the shape of the bed. Though my bunnies like to have the sides up on bedding so they can sort of hide in it, if all four sides are up, Jack, who normally doesn’t pee on bedding, will pee. I don’t know if he thinks it is more of a litter box or what?