I'll be curious to see the responses. I have three different kinds of birches, and all are multi-trunked. As expected, the younger trunks are shorter than the leader, but the leaf arrangement seems pretty static. The ones I have are a river, paperbark, and Himalayan. I really like the look of a multi-trunk tree, and would hesitate to cut out the smaller one unless there is compelling reason to do so. It might be worth a call to an arborist, although I have no clue what they charge for diagnosing. I would not put too much faith in the cold call 'tree specialist' companies. I get 1-2 unsolicited visits per year, and this last time I took the young man around to a couple trees I'm having issues with, and he didn't even recognize the tree species, let alone have any insight. I think he was selling limbing, removal, and pruning. Period.