Use "Channel mix" to swap alpha mattes for another image file

Beginners
1

In aftereffects or PSD you can use a third image as an alpha channel

Is there a way to composite to images with a third alpha?

3

The MatteTOP is the way I usually use a third alpha image to composite two other TOPs.

The default behavior uses the alpha channel of the matte, but you can set it to use Luminance or any other channel.