Unfortunately, in the current version of Family Tree Builder, and also on the website right now you can only look at smart match suggestions, and compare your tree with other trees that were suggested to you.
There is also an option to contact the administrator of the other tree.
In future versions, there will be an option to merge details of profiles on the trees with other trees.
Can I merge two trees? I know the other tree manager, and our trees only overlap at about the second tier at this time. We share the same detail on our paternal grandfathers, and that's it. Does the functionality exist to merge these trees?