The word aquaeductus can also be spelled aquae ductus or ductus aquae, possibly with the plural aquarum (see Lewis and Short). Spelling it separately in either order makes sense, as the aqueduct is a conductor (ductus) of water (aquae). But why is it also spelled together in classical Latin?
To be more precise, is there classical evidence for spelling aqueductus as a single word? If yes, what does that even mean? Word boundaries were not as clearly defined and marked as today, but they were not entirely non-existent either. Perhaps this is just a later editorial decision and has nothing to do with classical spelling, but it's hard for me to judge.