Alpha blending a texture?

My question is how to reuse the same alpha channel with different RGB textures?
I am doing a terran renderer. My thinking of texturing the terran is to use different textures representing grass, rock, etc. if one block changes from grass to rock, i want to draw the 1st texture and have the 2nd alpha blended onto the 1st.
I know RGBA mode will work, but to my understanding, a grass texture with different alpha channel will be a different texture, am i right? So how do i solve this problem?
Thanks in advance

maybe have a look at the tex_env_combine extension demo

I can’t link to that site. don’t know why ;(