Like the title says, I've got to blend two bitmaps together and I need to retain the destination alpha. I've kludged around the problem for now by re-drawing the original bitmap with this blender mode:

ALLEGRO_ADD, ALLEGRO_INVERSE_ALPHA, ALLEGRO_ALPHA

...which is an absolutely awful solution, so I need to either figure out a one-pass draw blender which retains the destination alpha, or implement the whole pipeline in a shader.

As a bonus question, how do shaders compare to blender options in terms of performance?

Blenders inherently work on all colour channels at once, so in order to achieve different results on different channels in a single pass, you're gonna have to use shaders.

Siege made note that you can only operate on source pixels though, so what you'll likely need to do is have a destination bitmap that's separate from the two bitmaps you want to blend together. The plus side though is that shaders are lightning fast and for what you want to do there shouldn't be a performance hit at all, even on low-end hardware.

*facepalm* I totally forgot about that function too, and I've even read about it before!