What happens if you hit a rolling modifier when you have advantage?
When you have advantage, you draw two modifier cards.

Then:


If none are rolling modifiers, you pick the better card. 
If one is a rolling modifier, you add its effect to the other card.
If both are rolling modifiers, you continue to draw cards until a rolling
modifier is not drawn, then you add together all drawn effects.



  Source: Page 20 of the 2nd Edition Rulebook, Advantage and Disadvantage section