Currently we split the RU allocation, ie, if both are 4RU, they get 2RU each. Combined with being able to have a photo next to the layout usually makes it easy enough.
However, I could easily see a situation where there is more than one item in a single RU, especially the rack mount kits that allow 2-3 pieces of equipment into a single RU.
Similar situation for the front vs rear of the rack. In that case, if necessary, you have two "racks" setup.