G'day Max,
It won't break the game, but as Adam says, it will have fictional diluting effects. Although simply having the players choose a different race but still be a Cleric is encouraging some narrative variation potential, I would stipulate that the players have to choose different options from each other at every stage of not only chargen, but when they pray for spells and choose advancements too.
I would also encourage (through provocative questioning) how those choices are conflictory with the other cleric, encouraging character tension and thus resultant drama in the fiction.
This way you will get two clerics from widely disparate faiths, with not only distinctive 'looks' and signature trappings / armour / weapon, but covering the broad base of cleric moves too, giving the party a strong (albeit tense) 'faithful' focus.
I can see a strong religious theme coming out of play, and a story full of moral ambiguity. your monsters are your arrows remember? Make them challenge the clerics, be a fan of the characters and allow them to shine.