Multiple Agent Types in Prefab Instance

So I'm building a generated city, as explained in another post. I have 2 agent types, basically the AI humans and the Player human (who can go places the AI cannot). Apparently (and please anyone correct me if I'm wrong at all here) each agent type needs their own navmesh.

Ok fair enough, but.....NavMesh Prefab Instance only allows one navmesh to be assigned to a prefab (ie tile square). So, um, how? Also, creating each Navmesh using NavMesh Surface (in the editor) *overwrites* the previous NavMesh - even if you rename it first! EDIT: it overwrites it even if renamed and in a different folder! So you can only bake one Navmesh per prefab?!?!

Not to mention the NavMeshes are automatically created in a default folder - why no option to create in the same folder as the prefab that's creating them? That would be much more useful for games with many prefab Tiles, such as mine. Also an option to bake all Agent types at once would be nice too!

Sorry, hair being pulled out here...