Create Stereo to Mono mix

Is there a way to mix stereo to mono? I tried Mix, and I can swap channels, but not mix them to mono. Left. / right to center and then center to right / left does not work (I think I get that). Crossfade also doesn’t do it.
Any ideas? Am I missing it?
Using phones, It’s much easier to get the balance and EQ right with a mono signal and then go back to 2 channel. Yeah, I have some mono sources, but there is other 2 channel material that would be better suited.

A couple of pictures tell more than a thousand words …

Et voilá - Stereo to MONO without clipping

Thanks!
I tried to do Left/ right to center and center to left/ right in the SAME mix panel. It requires two mixers, as you point out.
Works fine.
Much appreciated!