Hello! I don't know anything about separating a tree (though I believe it is somehow possible to split a tree), but if your concern is not privacy (i. e. the other people shouldn't be able to see the descendance of others) but relevancy (i. e. everyone should follow their own descendance easily) you could tell your relatives to go to "Options" and turn off the display of "Other" (i. e. aunts, uncles, their partners, their partners' ancestors, their descendants) so that the tree only shows one's own direct ancestors and descendants.
