You do this all the time in your car when you decelerate. Seems to work
just fine.

The question for you is: does your PTO have enough torque to operate
your saw? The current setup has a two-to-one reduction so torque is
doubled (ignoring losses). Going the other way, your torque is cut in
half.

Another question comes to mind. Will the shafts connect to the PTO? How
long do you intend to run the double speed and what is that speed? If
you run 440 rpm in and get 880 out, will this be enough speed for the
saw?

Paul