Still makes sense. One guy is not carrying two large backpacks into the location, it was one guy per backpack, posted out where he could see a signal from the people placing the bombs as they go for cover. The first guy put his bomb into position, stepped over to the area to give his signal, second guy put his bomb into position, is on the way clear while something happens, someone figures it out, shoots the upstairs spotter guy, upstairs spotter guy triggers the bombs before bleeding out.

Possibility #2, someone at another location gets the signal that something is up, the cops are onto them, they start popping the bombs, DHS is onto that too, and previously were trying to trace the cell phone locations but had not figured out the main guy was on a roof, since there is some tracking error on the programs, and the maps are two-dimensional, so they were searching ground level. After the first bomb, decision is made to shut down the cell phone system on a countdown basis, then second bomb goes off, then additional bombs don't go off, but the search is on to find them.

If they manage to get hold of one of the phones intact, they can figure out how many and which numbers it had been calling, which would probably indicate the numbers and locations of the other explosive devices.