how to avoid baking AO on specific components of the same assembly?

[Tonypezz]

how to avoid baking AO on specific components of the same assembly?
i have a large assembly and need to import all together, this is a game asset and some of the parts will be hidden/shown in game (replacable) .
In order to avoid baking contact shadows where i don't want them, I move the pieces apart but sometime it is just not possible.

to better understand, I don't want the contact shadow between these 2 pieces in the picture below but they both should be there:

how can I set that the lower piece should not get AO from other meshes?
I don't have a low and high poly meshes, what you see here goes directly in game.
Thanks