Texture-env + multitexture

Apologies if there’s a really obvious answer to this, but it’s starting to irritate me.
I want to use multitexturing to cross-fade between two textures (for smoothly animating sprites). I’d prefer to set it up entirely within the texture environment, since I’ll be using a variety of blend-modes, alpha-testing etc and I only want to write it once. I’d also very much prefer to avoid using any extensions (with the obvious exception of ARB_multitexture).

I’ve been trying to get different combinations of texture_env_mode, but they’re mostly very unhelpful; TEXTURE_ENV_BLEND in particular seems to be completely backward. Or is it?

Any suggestions / advice?

Thanks in advance,
Mike F