That niche is usually filled with something like a Stream Deck alongside your keyboard, though personally, I'd recommend MIDI controllers.

You get a lot of extra buttons very cheaply, using an open, easily-readable protocol. Get a label maker to put some labels on top of the buttons, and you're good to go!

In my experience it's a much easier option when compared to trying to say to your computer "key A has two functions, depending on which keyboard I press it on".