Hi!

If there are 2 asteriods in the line of sight when shooting, does the defender roll 2 additional green dice?

Just the one. Your attack is either obstructed or it isn't; there are no "levels" of obstruction that increase the amount of bonus dice.

The exception being a huge ship with EM Emitter.

Thx guys!

So when the Decimator hits the meta the same applies? 1 asteriod and 1 Decimator still yields 1 extra green die?

Yes. I assume Decimator has Tactical Jammer. But 1 dice either way