Mudbox :: Only Paint On Isolated Object?

I am painting a brick wall, the wall is separated into 2 objects, the bricks and the stuff between the bricks.

The problem I am having is that I isolate the stuff between the bricks and paint that gray but when I "show all objects" the gray color have transferred to the edges of the bricks. I have tried maps with different resolutions but that makes no difference.

How do I make it so that I can paint only on the isolated object?

One other way:I can apply 2 different mudbox materials, 1 to each object and create 2 diffuse maps, 2 bump maps and 2 specular maps. But how do I combine the different map with each other?

The objects are going to be combined into 1 mesh that uses 1 diffuse map, 1 normal map and 1 specular map.