tried to trade with myself on a single pc, had to do some save copy-pasting and create a second account. in the end still didn't work because of the games pausing while in background mode.
I did manage to accidentally merge my 2 savefiles giving my character 10 minutes into the game all badges and party pokemon from my lategame save. dont really understand how to recreate this but with some creative fiddling this could give an option of "trading" on the same pc.