Hi XXXXXXXX
You may have to try using two different litter boxes. Sometimes cats can become very fussy about sharing. This can happen even after a long stretch of sharing the litter box.
The other thing that comes to mind is one of them may have an infection of the urinary tract or bowels that causes them to not be able to hold it long enough to get to the litter box.
Hope that helps,
Kerry
Cat Owner
RN, Cat owner