Okay, I must be slow.

Someone in here said you can stack powerups to use in sequence, but I cannot manage it.  I've got a yellow and a white, but if I grab the white first I can't shoot after flying, and if I get the yellow first I can't fly at all.  What am I doing wrong?

'Stacking' probably isn't the right word for it: Mi can't absorb more than one power-up at the same time. What she can do is absorb another power-up while she's actually using the first one.

In your case, you can use the yellow power-up from under where the white power-up is, so that you'll be absorbing it as you fly up, and will be able to shoot at the end of your flight.

'Stacking' usually refers to absorbing the same power-up twice in a row, though (since they respawn more quickly than they expire).

Actually, AA is not entirely correct. Yes, you can pick up powerups while you are using one, including getting the same powerup, but while holding the powerup, the same rules apply as when you are not using a powerup. It will cancel itself if not used within x seconds. So make sure that if you fly and you grab the white powerup, that you abort the flying quick enough to still be able to fire the white powerup before it dissipates.

Powerups can only be used one at a time. Once canceled or the effect expires, they are lost until picked up again. The only way to get around this is to 'stack' them, but you can only really 'carry' multiple powerups with you, rather than actually have many effects at once. This is useful but the powerups you obtain will always expire after a short time and become useless anyway, regardless if it's the only one you have or not. So the only way to use both properly is usually to cancel the first slightly early, and that's tricky, especially if you're not used to it. The reason this doesn't happen unless you cancel it is because most powerups (flying ones, especially), take up more time than the time limit for activating the powerups. So basically normally you'll end up losing the ability to activate anything else before the flying's done, and it will seem like you can only use one powerup. So you can try this:

Collect powerup A
Activate powerup A
Pick up powerup B
Cancel powerup A
Activate powerup B.

Which would result in you having done feats impossible otherwise. Anyway, by this reasoning, due to the time limit on activations, there's only but so much stacking you can do with the powerups, because of the time it takes to get to another one, and cancelling/activating promptly. But using two is very possible.

To get back to your situation; you can't fire the white one while flying with the yellow one. You can still probably use it after flying, but not during. If you cancel the yellow one to fire the white one, you'll fall.

Hmm, okay.  Thanks!