Hi Naomi,
In general, if you have many to many relationship the you use 2 dimensions and ask user to select members in both dimensions (because there is no fixed algorithm to select one depending on another).
In our case we have the same scenario with TITLES and ENTITIES (we are publishing house). The same title can be present in multiple entities. We use 2 separate dimensions and we don't use concatenation.
Hope it's clear.
B.R. Vadim