If you tall jump using the movement stick and on the first and second frame use any c stick smash besides up, it automatically uses your second jump.
Do you mean jumping with tap jump, holding up on the control stick, and then flicking the c-stick (set to smashes) in any direction but up while im jumpsquat? Jumpsquat being the animation your character goes through before leaving the ground.
If so, that's not a bug. The way Brawl reads c-stick inputs it treats them as a control-stick input in the corresponding direction + A for one frame (if set to smashes). If you're still holding up it then thinks you've moved the control-stick up again and thus triggers a jump (holding the control-stick up can buffer a jump for up to 4 frames if jumping was impossible the frame it was activated.) You can use this behaviour to get double jumps on the first airborn frame and for example
dj-land with Yoshi or Peach. I explained the mechanics behind triggering the instant DJ at greater length in the thread I just linked.
If you meant something else, I don't quite understand what that is.