Originally Posted by
catocony
The OP will only go through TSA once, and that's on the return from Brasil. After clearing Immigration and Customs in ATL, he'll have to go through TSA to get to the flight to YYZ. He doesn't need a passport, I would hope that TSA can figure out a Canadian driver's license. If not, either passport will do, as it's not important beyond matching a boarding pass to some form of photo ID for TSA purposes.
TSA doesn't bother about which passport you show, just show the passport of your please to them.