One foot, two kicks in the time you usually play one — that's the bass-drum double. It's the technique behind every "boom-boom-bap" hip-hop kick, every "ka-ka-snare" rock fill, and the rapid kicks under a metal verse before the second pedal comes out. There are three popular ways to produce it, and you don't need to master all three — pick the one that feels least awkward and develop it.
The heel-toe (rocker) technique pivots the foot like a rocker: the heel drives the first stroke (foot tilted back, ball of the foot up); the toe/ball drives the second (heel rises, ball drops). Two strokes per pivot, controlled but limited in volume.
The slide technique starts with the ball of the foot near the heel of the pedal board; the first stroke is played there, and as the beater rebounds the foot slides forward toward the toe end. The second stroke comes from the now-forward position, where the leverage is greater. Slides feel slightly awkward at first but produce two equally loud, equally clear strokes.
The swivel technique uses a side-to-side rotation of the leg: the first stroke from one angle, the foot pivots around the heel a few degrees, the second stroke from the new angle. Less common today but still used by metal drummers for sustained doubles.
Pick one. Frame the others as alternatives you'll come back to. The exercises here are agnostic about technique — they just ask for two evenly-spaced kicks in places where you used to play one.