Much is dependent on the role that the new step takes. If they try to be a a help-mate to their spouse, but not take over parenting a child, chances are that it will work. There are a number of parents on this forum who get along well with all the respective X's and their spouses. It depends on how much they have gotten past their anger and what they choose to focus their energy on.
If the mom takes the CP role to heart in which she is to faciliate a relationship with the child and his/her father, then she may accept a step who will help get information to the dad. But, if the mom feels threatened in her role as mom, then chances are that it won't work.
It all depends ...