There's a reason why lathes with live tooling have huge horse power motors. It's not that you're ever likely to use 30+hp while turning, it's so they still have enough torque to maintain a stationary position.

Given you're working with wood, the forces involved shouldn't be as high as metal, but you can still calculate the potential forces. If you know how big your live tooling motor is, and how much torque it can produce, you can then calculate the potential maximum force on the cutter edge. You can then take that force and calculate the torque the spindle needs to produce to hold steady (plus add a safety margin).

For ideas, search out the In-turn (there's a huge build/development thread on the mach support forum), which is a dual speed 4th axis add-on.
However, having a suitably geared motor only connect for positional work is a valid option. At low speed, having the high speed motor still connected shouldn't be a problem. It will add a bit more inertia, but I'd be looking at keeping the low speed at under 60rpm, so that additional inertia is going to be minimal, and reduces the level of complexity you need for changing speed.
The big problem is how you handle backlash in your positional motor drive system.