On my question, yes I'd be treating them as two separate amps that happen to share one case with two sets of inputs, two volumes and two headphone sockets so umbilical plugging isn't any problem. I'm not looking at a complex solution, just a simple and neat one.
Switch off power, swap umbilical socket, power on... Is that reasonable?
Edit:
I will be feeding both from a Pico DAC so parallel input would be nice as my source will be USB for 90% of the time