Sadly, I don't believe this is possible - as far as I know, you can't connect two seperately created family trees (at least not with this website), and you don't have any way of visibly distinguishing between two "family lines" in a tree (i. e. creating the "Mayfairs" and the "Bankstons" who occasionally intermarry and coloring every Mayfair family member's box green while the Bankstons are blue).
The only solution would be to just create one giant tree in which you add all family members of both families. Size is not a problem (you can have literally thousands of people on a single tree), but large trees are usually less clean to view (though you can of course play with display settings).
If it is only about viewing two trees at the same time and you don't need them connected, maybe just open two windows in your browser and display both trees at the same time (or open one on your Desktop device and another one on a mobile device).
