Children who split time between homes often hold through the transition. The handover itself is unsettling, the first evening in the other house is unfamiliar again, and the after-meal sit may not exist there. A poop that was due on Friday arrives, hard and painful, on Sunday.
Differences in approach matter more than parents expect. If one home has a footstool, books and no pressure while the other has 'you're not getting down until you go', the child does not average the two. The toilet becomes unpredictable, and holding becomes the safe default everywhere.
Parents who are co-parenting through conflict can find that poop becomes evidence: accidents after a weekend 'prove' something about the other household. The child feels that, and it adds shame to fear.
What works is small and boring: the same short routine in both homes, the same words, a shared log, and an agreement that the pediatrician's advice is the plan. It asks nothing of the adults' relationship beyond a page and a handful of texts.