Vainly searched whether or not this has been asked before.
Joining text files: suggestion to add an option to add xx lines between the texts and as well as some kind of separator.
(I know I can install a 3rd party tool for this)
You could put what you want into a file and include it between the files you're joining.
A script could also do it.
The Split and Join tools themselves are more aimed at splitting and recombining binary data to work around things like media and attachment.size limits.
Thanks. Bad luck this time. I wasn't aware it was meant for binary data.