How boring, but if you must have both it's so easy. A stereo pre/amp or intergrated unit being fed your 2 ch sources, and your normal multich setup. Just change out the L & R speaker cables to switch between, easy peasy.
But why?
The myth of dedicated stereo sounding better than multich is just that, a myth.
99.5% of the quality of your rigs reproduction will depend on the quality of your speakers and your rooms tuning.
In the majority of real-world cases a decent AVR based rig will whip the stereo if it's DRC software has been applied correctly.
Plus the AVR offers access to all the multich sources available today.
With the explosion of immersive music being offered by the streamers and on disc, it's very possible that the stereo-only world is coming to an end. As long as Joe Sixpack can be convinced "spatial" audio is an improvement over stereo and he only needs a $199 crapbar to enjoy it, that will leave only the teeny tiny audiophile world to complain.
BTW, when I had my home up north with a number of rooms to listen to music where I wasn't sitting in a sweet-spot to enjoy, I had a number of mono rigs running. The kitchen, basement, and garage had some killer rigs using old classic HiFi gear. Who cares about stereo when your cooking or building an engine?