Daniel, I personally wouldn't be all that concerned with the structural fidelity as long as it looked reasonable.
Lou, coming from you, that concerns me. Are you referring to the ability to attach one part to another because of the material? I was simply thinking of not installing one truss on each model and joining them in the center somehow, even if it was a mechanical joint or heat welded and then hidden. Maybe a styrene I beam of significant height inbetween.
The bridge will only be about 1" off the water surface when complete.