I have a vise with (replaceable) wood faces on my big heavy workbench. I clamp down on the freewheel such that the teeth on the big cog are biting into the wood and the small cog is above the vise, hence needing only one chain whip. If my meager upper body strength proves insufficient, I whack the end of the chain whip a couple of times with a mallet. This has always worked for me so far, even on very old, very high mileage freewheels.