My own combi machine is saw/spindle moulder - planer is separate machine - but all 240V/1P so haven't had this problem. I guess that you normally change over between motors while nothing is running, so I guess the usual kind of VFD would work.However, I don't really like the sound of it, thinking about how you would isolate the motors for blade/cutter changing/adjusting. VFD that is happy to be to have its output switched live seems like the best way to go.