If each day of the move involves a different crew, I tip that day. Hoping the first crew informs the second crew that this residence tips, you may get even better service the second time around. In your instance, packers may not be the same team loading or unloading.
I tip $5 an hour per person. Half a day is $20 and full day is $40.
Now I got a question for you; how much to tip a white glove delivery? I am having a new fridge delivered. They will move the old fridge to the garage for me as well. The fridges will require being brought in the back doors since they are french doors, 60" opening and a fridge is quite a bit larger than any other possible access. Maybe 200' from driveway to kitchen this route. I'm thinking $10 each for 2 guys?