I'm with Luvya -- I don't think it's a great idea to make two different amps. My philosophy is that the more a headphone amp stops interfering with the music and starts passing the input signal to the output untouched and enables the headphones to sound as good as they possibly can, the better.
This isn't to say that I'm into "colorless" amps. So far, I haven't found anything that's truly neutral. So, it's possible to have two amps of roughly equal performance ability but with a different sound. I guess it's also possible that one could sound better than the other for some things.
From my biased perspective, it seems to me that a pair of META42 boards, one inverted over the other, would fit together well. For instance, consider two of these in a Serpac H-67 case, with one mounted so it sits on the bottom of the case and the other fastened to the top half of the case. You'd be somewhat limited on component height compared to putting just one amp into the same case, of course. But it'd work.
I suggest a 4PDT toggle switch to change from one amp to the other: wire the input jack to the common lugs of 3 poles of the switch, and run wires from the other lugs to each amp board's input. Then use the fourth pole to control power to each board. Wire both amp boards' outputs to the same output jack -- I don't think the amp that's powered off will interfere with the signal from the other amp, even though the idle amp is technically in the circuit all the time.
Another way you could do it would be to hook up just one amp board's power supply and make that feed both boards, but set the amp toggle so that it sends the power supply's output to only one board at a time.