1. The single stroke roll is by far the most useful rudiment for double pedal work.
2. The state of the art right now in double pedal playing requires having an excellent single stroke roll that can be used at any time, in any circumstance, as well as the ability to seamlessly incorporate hands and feet together so that all 4 limbs are used in beats, fills etc.
If you just got the double pedal you need to spend about 3/4 of your time on the single stroke roll and the rest on hand foot combos.
For hand foot combos start with RH, RF, LF as a triplet. get it fast and smooth.
Go from there and work on getting a seamless flow between hands and feet.
Good luck, you have years of work ahead of you.