Sure - that’s a function of the transmitter to handle the mixer and logical switches to operate the two RC channels in the required combination. Each relay/GPIO output is assigned to an RC channel of it’s own.
I know it’s easy enough on any OpenTX or EdgeTX transmitter. I’m not familiar with mixing on some other radios. I did some mixing on old cheap radios, but that was a couple of hundred years ago now
It used to be common to set up 6 flight modes from a 3 position switch and a 2 position switch - so it’s a similar theory to that.
Know transmiter mixing but here I’m looking for something like in-firmware mixing. But yes, I will go with transmiter mixes if there is no other solution.