I have a shop-grade freewheel vice tool that fits into a large bench vice to immobilize the freewheel. This allows the use of a single chain whip to unwind the one or sometimes two threaded outer cogs. Sometimes these cogs are threaded on so tight that you need to liberally apply penetrating oil days in advance, and then hammer on the chain whip - hard. It may not be possible to generate this amount of force/impact with just two chain whips.

Using a solid block of wood that fits into a bench vice, then you could immobilize the freewheel by applying some wood screws around the periphery of the outermost cog. This is basically how my freewheel tool works, except mine in made out of metal, and has multiple holes for the bolts that are used to immobilize different sized outer cogs.