Will the Tonewood amp work with my Taylor 812c with LR Baggs Anthem pickup? Thanks!
That will work just fine. You'll need to blend out the Mic signal to prevent feedback.
The arch can be tricky, but you can offset the TWA if you need to.
On my Taylor, I just scooted my TWA closer to the output jack and was able to mount it flat on the downward slope of the arch. I've also seen them off to the side and they still sound great there.
Your Martin will work fine, too! Just be sure you're setting up the guitars individually in Global Settings> guitar so you don't have to constantly change the settings when you switch.