I'm not 100% sure about it, but I think its 1:3888.
What I did was treat each die as an independent roll, not all relating to each other, because when drawing a random number from a set (like a set of dice) it doesn't matter if you draw (roll) all numbers at once or if you do them each separately. So you have (1/6)(1/6)(1/6)(1/6)(1/6), to get 1:7776. Multiply by two, since you said to roll twice, and you get 1:3888.
Again, I'm not completely sure about it, but that's my best guess.