I think the fallen really aren't that different than us, only that during their collapse the traveler left where in ours it was crippled and remains a source of light. They want the traveler back and we are in their way, but outside of that, I don't believe we would be fighting them. They definitely are our enemy but I think both would recognize that the hive and vex are a bigger threat, than each other. So in the heat of battle, two enemies see the larger mutual threat, and for a moment have a truce.