I see no reason why it shouldn't be a once a turn free action to switch active hands, there are already multiple ways to use both a two handed weapon and shield in a turn, right now the kasatha and skittermander just lose an action to do so, it would still prevent two weapon flurry with two handed weapons, swinging with two different two handed weapons in one turn isn't a huge advantage over swinging one multiple times, and it still limits the effectiveness of increasing the amount of hands a creature has.
Feats could count a single weapon as effectively being in "one hand" for purposes of things like two weapon flurry