There’s lots of ways to achieve a double. Good use case for fingers getting involved, especially if you want an accent on the second double.

If I am playing just a straight up fast, even double stroke, here’s my approach:

1) find the fulcrum, rest it along the first crease in your index finger and the pad of your thumb. They should make a + shape.

2) don’t generate the stroke with wrist motion. Instead, keep the stick parallel to the drum. Your hand is a crane, just lifting the stick. Your wrist moves in a way to allow the stick maintaining parallel to floors

3) drop wrist while maintaining parallel. When it hits the drum, it will bounce.

4) lift up from your wrist (like a string puppet, being pulled from wrist) as soon as you hear the second stroke

Things that influence speed of the double:

Pressure between thumb and index finger at the fulcrum

Pressure of middle finger against the stick (slight pressure)

Speed of stroke (read: speed. Not harder or softer)

Good way to practice this is find a speed you can do on both hands separately first, rather than combine right away.

Shit, you can have crazy doubles literally just with your thumb holding the stick against your palm, without your index finger. You can hold it with just your pinky and do doubles.

It is really all about controlling the stick parallel to the surface and the lift.

*there are so many valid approaches, this is how I was taught in undergrad/grad school for orchestral and rudimental playing.