Hiya, I’m using Blender 2.49b (and I have to use it),
I have a few parts on my model, where two different objects render at different times, but are sharing the same part on a texture sheet. In this particular case, one model is a frame, and another is the backing. Right now, the part where the frame is on the texture, is black, and that is because the backing renders it’s ambient occlusion on top of the frame. Is it possible to reverse the order Blender renders here?
Some imagery to support our imagination would be nice.
However, AO isn´t calculated over something.
As the name says, where something from the ambiance occludes, it´s darkened.
Or more specific, for each point in the scene where a ray intersects with an object a sample of rays is shot spherical in the scene and the more rays are terminated by intersection, the more the point is occluded and darkened.
So there is no order. The scene is evaluated as one. You can´t check for occlusion of a point without checking against all objects in the scene else the result would show… something. 
arexma is right. The pixel value on your AO map is generated by computing 1 - number of hits / number of rays. So the order doesn’t matter. During rendering your image only gets darker, never lighter.
The easiest way to fix your problem would be to either fix the overlaping on your uv map or generate 2 ao maps for each object and combine them later on by hand.
Or simply paint over the dark area in gimp/paint mode in blender.