It appears to me that the two bebackpacked individuals are together. The second one loses his backpack in the footage. I assume the first has the first bomb in his backpack.
I've watched the footage a couple of times. I don't see either of them losing their backpacks. What part of the video are you seeing suspect #2 without his backpack?
This is from the FBI Youtube account and it's linked to on FBI.gov.
Guy in the white cap is the one who they say placed the bomb.