Could it not be something as simple as it can follow one river down, get to the coast, follow it round and then go up the other river.
Sailing allows trading on Coast's and river's after all, no?
I don't have sailing
Dan's explanation is the only sensible one I can think of.
I didnt think you needed sailing to connect resources by river. Well not in the games i play anyway.
More example,
in the screen shot below, my city has access to corn.