Add To Your Selections Through The Channels Palette
If you’ve got the Channels palette open and you have multiple saved Alpha channels, you can load any Alpha channel as a selection by holding the Command key (PC: Control key) and clicking directly on the channel’s name. This instantly loads the selection. An even better tip: If you hold the Command key (PC: Control key) then add the Shift key (making it Command-Shift/Control-Shift) and click on another Alpha channel, it adds that to your current selection. You can keep adding more selections to your original selection until, well… until you run out of Alpha channels.