It will take 2-3 bill cycles for your rebate to arrive. If you cancel before the rebate is approved, your rebate can (and likely will) be invalid.
When I did one of their rebates in the late-summer it took almost the full 8 weeks for it to arrive (6+ weeks to get the approval confirmation and another week or so for the prepaid MasterCard to arrive).
If you are going to do this, pay off the phones AFTER you get the approval email (since the rebate is conditioned on EIP) and also put in a request to have them both unlocked. If they ask why, tell them you are going on an international trip and need to be able to use a local SIM for low cost, high speed service. Phones must be used on their service for something like 45 days to be eligible for unlock.