So if a normal non magical broken sword can damage Sauron what do you think a lightsaber would do? Plus i haven't seen Sauron move anywhere near as fast as Sidious can so to think he could dodge or block the lightsaber strikes is just silly.
And with such an great speed difference it is very unlikely that Sauron would ever hit Sidious in return let alone mount any kind of offensive. That's not even bringing into account Sidious' other force powers (lightning, percog, force crush, force stroms, force push/pull, i can go on but you get the picture)