Older brother joined jihad in 2009 (according to Uncle). Older brother had 6 months of terrorist training. As far as we know the younger had none. The old brother ran into gunfire, the younger surrendered. The older brother studied engineering, the younger was pre-med.
Perhaps you're assuming that this was a two man operation. I highly doubt it. Too many unexploded bombs were found in the vicinity for two guys to have planted them all.
Then there's the mysterious case of the Saudi (Al Harbi?) who was injured in the blast, but is now suddenly on a fast track to deportation. Lots of other loose ends have popped up as well, and are quickly being tied together as we speak.
Point is, these youngsters didn't commit this atrocity all on their own. Terrorists never do. There's always a hidden command and control network behind them.