Most of the time there isn't a problem with switching one hand off (free action), doing something with the free hand and then put it back on again later (free action).
The problem comes if you need the free hand when it isn't your turn (which rarely happens but sometimes does). Since you aren't wielding the two handed weapon you can't make OA with that weapon, you can still punch them with your free hand however, or even count the two-handed weapon as an improvised one-handed weapon.