I could be wrong but I don't think at this time there is a way for someone to combine two seperate trees that they have made. To have ones Mother and Fathers family together on the same tree one would have to put them all on the same tree to start with. Maybe this is a feature they might consider doing as having to do that would be rather tedious having to re-enter all the names/info unless the tree was quite small or they hadn't gotten far into creating their trees.