The effects are not cumulative. The language in the rules doesn't specifically say this; though, there is no mention anywhere about rolling 2 dice for 2 obstacles. Here's what's in the rule books:
Obstacles represent space features that are difficult
to fire through. When measuring range during combat, if the edge of
the range ruler between the closest points of the two ships overlaps an obstacle token, the attack is considered obstructed. Because of this
obstruction, the defender rolls one additional defense die during the “Roll Defense Dice” step of this attack. Remember that range is always measured as the shortest distance between the two ships’ bases. The attacker cannot attempt to measure range to a different part of a base in order to avoid obstructing obstacles.
Here's a similar situation from the FAQ:
Q: After a ship moves through or overlaps more than one
obstacle, does the ship suffer the effect of each obstacle?
A: No. His opponent chooses one of those obstacles; the ship suffers the
effects of the chosen obstacle.