I believe only one of them will be blocked. The other will be entirely unblocked. Because it's all for one of them and none for the other. Your opponent can't block half and half for each one.
September 17, 2015 11:01 a.m.
General rule to follow in these cases: Obey as many of the requirements as possible, without violating any of the restrictions.
In this case, You choose which Breaker to block, but all available to block creatures must block at least 1 breaker.
The opponent will never decide how your creatures block unless an effect like that Brutal Hordechief says so.
There are some corner cases like the fact that Guardian of the Gateless must block both, but those are likely not to come up.
September 17, 2015 11:08 a.m. Edited.
Boza has the most correct answer. Each of the defending player's creatures must block one of the Breakers (the defending player chooses which Breaker each blocker will go to), and any creatures somehow able to block more than one attacker must block both Breakers. The blockers don't have to all pile on one Breaker. They can be split between the two of them as the defending player sees fit.