Hi Franz,
the most simple way to face an engine to itself once, is using the demo function. But of course this achieves a single game.
If you want a real tournament mode (to achieve several games in a row, eventually with several participants,...), you can try editing the prepared .at file and substitute any participant, copy/pasting the one you want to enter twice. Looks like the limitation is only at GUI level. The expected games are played, you get the pgn files of course, on another hand the tournament table/results are not relevant. I guess that's why the GUI prevents so.
MfG, Eric