if the two TTrees have the same number of entries you could add one TTree as a friend of the other, see e.g. TTree::AddFriend.
This essentially “pastes” the TTree together horizontally, making branches of the friend tree available as if they were branches of the main Tree.
Hi thank you that looks like what I wanted! I do have a question though - both of my TTrees are created with the same names and the same branch names (I have no control over their creation unfortunately). I can’t see an easy way for me to change the name of the branches I want to add. Is there a quick way to change this when moving the branch over?