Just cat them together. I am not sure what you mean by trustable?
If you have spaces in the headers (e.g. words separated by names) make sure to change the space to _. Most aligners will drop things past first space when reporting alignments (e.g. bbmap is an exception and will keep the entire name by default unless you use trd=t). You may get confusing results if the part of name up to first space is common to more than one record.