If the girl is correct, I'm curious why no explosions on their part?
that's a good point, suicide bombers work alone, they usually don't have some 2nd "guide person" without a bomb working along side them. these simply could have been two people near the location where it went off, who ran like hell when it happened. and they can't run into the stadium, I imagine the entrance/exits are gated and have security.
Speculation: They were to set off the second bomb after the responders were on scene, probably remotely. Since the first bomb went off well before they intended it to, and there was no need to stick around to do that since the only casualty was the first guy. Of course, if it was a terrorist, it's possible that even the first bomb was not supposed to be a suicide, but maybe something set off by a cell phone. (Dumb thing to do in that environment as football stadiums, and campuses in general, are so full of cell phone signals it's amazing it doesn't cook the birds as they fly by.)