Could you elaborate a bit on that? When exactly does the "Split tree" error occur? Are the two types of parents already assigned and you want to edit both pairs? Or are you trying to assign them?
Generally speaking, the "Split tree" error occurs when your changes would result in one person losing their connection to everyone else on the tree. To prevent this, you could either try deleting and re-entering the person in question, or maybe add a (wrong) working connection to the tree (i. e. you make the person in question a sibling to someone on the tree, do your work, (re-)connect them with however they really are supposed to interact with the tree, and then erase the sibling relationship you just added).
