Mainly the skip asks the sweepers do both sweep actions, but sometimes the skip asks a single player to do action.
What is the difference between the sweeping to choose one rather than another sweeper?
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It's affecting the curl of the stone.
So depending on who sweeps (or sweeps more than the other sweeper) you can basically steer the stone by giving it more or less curl... so if you need a lot of curling only one sweeper takes action until the stone is on its designated track, while with two sweepers working equally the stone just goes more straight forward.