Hi,
The best solution is to use AddFriend which should work for most circumstances. Merging laterally two files (i.e. was you are looking for) has not yet been implemented as a core feature and thus you would need to do it by hand (i.e. construct a 3rd TTree with the same branches as the other two combined and explicitly set the address of the branches to the same (respectively) addresses and the loop over the tree entries to read and write (i.e. GetEntry and Fill).
Cheers,
Philippe.