I'm assuming you have to get the handle down to pretty much finished dimensions first? Trying to figure out if this is something done before or after scales and pinned and glued up. I'm thinking after?
Yep you would be one millisecond from being very angry if you mess up and hit the ricasso. This is where the lucky people with speed control grinders are better off. My grinder has two speeds, off and WAO.
I've done this a few times. You could do it with a small sanding drum in a dremel. Better control and you can see what you're doing. Personally, I would do it before the scales were fastened permanently.
There in lies the difference in skill levels. If I did not attach the scales before I scalloped them there is no way they would be symmetrical. More than likely I could Temporarly attach the scales together, scallop them then separate them but something would derail in the glue up. Been there done that, threw the knife in the pond out back...
I tried it once, with no planning on a small wheel and it did not turn put good. My problem was they did not align one was pointing toward the spine and the other down. That is what I need help with how to get them both running down the center of the handle. I was thinking of getting my tool rest aligned to my small wheel and resting the spine of the knife on the tool rest so the scales contact the small wheel at the same elevation. I'm going to play with it and see.
It's a wood carving chisel that has an oblong, elliptical shape, the contour of which looks like the shape you want. I'm not a real big fan of power tool s at that stage of the build... Too much can go wrong way too fast!