First: If you complete a daily (DON'T TURN IN) you get the credit, without turning them in. There is a Achievement which is about "do all dailys". And since you don't have to turn them in, you can do them, complete them, get credit, abort them and do the other one for credit. => twice as fast.
So there are many Quests achievments which are only obtainable, if you have a specific daily. As you can accept both daily quests, you can do both specific achievments for each questline. => twice as fast or even faster.