That sounds plausible.
Even IF there were others involved, they must have been only remotely involved and didn't care or else didn't want to GET closer to these two. Maybe the outsiders felt these guys where just TOO amateurish.
They are probably laughing at the bombers right now. Those two brothers were cut for bait... and truly expendable.
Thanks. I can poke holes in my own theory, but I also think Joker and Tammy were amateurish. An event like the Boston Marathon would have cameras EVERYWHERE. Plus there is the possibility that they may have wandered by an explosives sniffing dog and been found out. One thing they did do that worked in their favor was to wait until the race was half over. I think that the security personnel may have been less vigilant by then.
That would explain their bumbling grand finale. They were set up as useful idiots.