How can I draw an alpha-channel, or clone one texture over the other in Texture Paint mode in Cycles? I'm looking for the texturing method in Cycles, similar to the texturing method in Blender Internal Render.

I have two textures, that have been Project from View-ed from two different angles. Now I want to erase some parts of these textures to make one texture seamlessly merged with another. 

How can I draw an alpha-channel for the textures in Blender (in Cycles, using Texture Paint mode) or how can I clone one texture over the other one (in Cycles)?
This is not cycles or Blender internal dependant, this is just texture paint mode. So the principle is the same. You can clone the same way.

You can also "erase alpha" (blend method in the "texture paint mode" tools panel). But this will not "unclone" the original textures.

If you want to erase part of one texture, clone again the other one instead of erasing using alpha.