Possible issue when cloning trees

Just to clarify, I am of course willing to accept we (i.e. in NA62) are going about this problem the wrong way. It is still puzzling that one can read a TChain, using multithreading, as I described as long as the chain consists of only one file, but trying to read multiple files leads to a crash.

This being said, I would also like to ask in parallel, what would be the proper way to correct this? Assuming, as I wrote above, that the input structure I am reading is quite complex and I want to be able to handle multiple threads.