KarnageT You have to select which tilemap to draw on using the "pointer" tool (normal layout view selection). It can be cumbersome when tilemaps overlap each other. You have to lock/hide all the layers except the one on which the tilemap you want to edit is on, otherwise it'll select the top-most tilemap.
RootKernel Multiple instances of the same tilemap won't duplicate the tileset in-memory, so no, it shouldn't be an issue performance wise.