Blit only alpha component

Hello,

I want to make hole in one surface by blitting aplha from another surface on
that surface, so when I blit first surface to display aplha mask from 2.
surface apears transparent. All my surfaces are per pixel alpha. So basicaly I
need to copy alpha component from one surface to another one, but using Blit,
not by locking surfaces and going through pixels, since I noticed this is too
slow even if my both surfaces are in sys memory. Is there fast way to do this
(if its not possible to blit) because I must do it in runtime (I cant do it in
load time).

Thank you in advanceā€¦