@KN4THX Thanks to you, too. Most (all?) Linux graphical desktop environments can output their screen on multiple outputs with different resolutions. With the oldest and (still) most common X.org display server, this can be done in shell scripts by the xrandr tool*. But Retropie comes without X.org or similar display server, and I also remember reading somewhere in this forum that the Pi 3b's hardware can't output different pictures at the same time (e.g. via hdmi and composite).

* edit: I don't know if tvservice can do anything similar on the Pi.

In addition to these obstacles and unknowns, I would prefer an external solution to free the Pi from any additional cpu or gpu stress, and (with less priority) to be future-proof from significant changes in the hard- and software.

That said, if there is a software solution for cloning the output with different resolutions without slowing the Pi down significantly, I would be happy to try it. :)