Using alpha from another texture?

Hi all,

I’m currently having 2 textures, A and B.
Both A and B are RGB textures without alpha values.

B had what I intended to be A’s alpha.

I wondering, how do I make B the alpha channel of A?

I tried multi-texturing both A and B together. Still, both of them doesn’t contain any alpha channel.
Texture A and B are both updated frame by frame from different parts of the rendered scene using glCopyTexImage2D, so I can’t have their alpha channel already done. Is there anyway I could perhaps use glCopyTexImage2D directly to get the alpha to A?

Thanks again.