It is both.
To do a double stroke, you are making one motion of the hand (or wrist to be precise) just like a single stroke. But, you use the bounce from the hit and snap your fingers without making a full wrist motion to get the second hit. Both the hits sound same because in the first stroke, the stick is hitting from a greater distance due to the full wrist motion and in the second, you have the force from snapping the stick with your fingers.
Now take a single stroke roll and use doubles as mentioned above on each stroke and you have a double stroke roll. For someone who is looking at you doing a double stroke roll, the wrist motions would look the same like when you are doing a single stroke roll.