I think it more likely that there'd be a third town (or just a fortress) occupying the high ground between the river-port towns. It would be able to control all the traffic passing between those two, and probably also become militarily dominant due to it's secure location. As it's political influences grows, it'll be able to attract more people to it and become ever larger and more prosperous, untill it eventually links with and absorbs the other two towns.

A city formed in this manner could have an interesting layout. It would probably be long and thin in overall shape, having grown around the road connecting the rivers. It would also have a bulge at the center, one that would be distorted by the shapes of the hills/ridge. The ends that hug the rivers would also be wider, possibly spreading quite a distance along the riverbanks, perhaps even extending across the rivers via bridges.