[avhon1]

Lots of 2-key keyboards exist, and are even for sale inexpensively. https://duckduckgo.com/?q=2+button+keyboard&t=ftsa&iar=image...

Not very convenient for having both buttons near your thumbs, though.

I've been trying out the related SwyftCard for Apple //e on the izapple2 emulator.

https://github.com/ivanizag/izapple2

  ./a2sdl -model swyft

It actually works really well; it uses the left and right alt keys as the open and solid Apple keys, which the SwyftCard uses as forward and backward leaping. It's quite easy and comfortable to use!