Definitely doable, but not without the potential for some issues.
One thing to look out for is amplified sympathetic vibration, since you'll be using piezos for one or both necks.
Piezo pickup systems will pick up vibrations through in the body, some worse than others, and can amplify the sympathetic vibrations of strings on the neck you aren't using, even when the pickup on that neck is bypassed.
If you aren't familiar with sympathetic vibrations in double neck instruments, basically the energy of the strings you are plucking will vibrate the body and make (some of) the strings on the other neck vibrate, that vibration will in turn go back through the body and be amplified by the pickup on the neck you are using. Sometimes that sounds good, like a delightful ghostly sitar accompaniment; other times it sounds bad, like someone playing an out of tune ukulele in the next room. It can be more or less noticeable, depending on how hard you play and what kind of amplification you're using.
That's why John Paul Jones has those little flip-down nut mutes on his triple neck 6/12/mando in the picture below. They also prevent stray noise from your arm accidentally hitting the strings on the unused neck.
View attachment 3101030
Connected to that issue, what preamps and switching system will you be using? Big polyurethane U-bass strings and little metal mando strings will produce very different signal levels, and need very different EQ, so you'll likely need two different preamps and a way to quickly switch between them. Will you be playing both through one amp, or splitting them out into two separate ones?
