You are overlooking one slight problem when putting twin motors into a usually narrow hull of a paddler.
The length of the motors, plus gearboxes, plus room to fit the belts on, plus support for the individual shafts.
You will most probably have to go for the offset method as shown on the sketch.
This is the way I did my Glasgow hull, and that is a beamy hull for a paddler. I used toothed belts for the drive system, and as mentioned by others, they require less tension to be applied, whereas normal ring belts have to be kept in tension all the time, and put undue stresses on the inboard shafts. I suppose the hobby chain would do just as well, but I have never tried it.
It all depends on what you want the model to do. If just to sail around and nothing special, then a solid shaft is fine. If you want to turn on the spot and have super maneuverability (totally out of scale, but looks great on the water) then independent is the way to go. But really, you need to do it electronically with a rudder/motors mixer and independent matched speed controllers, trying to do it manually is a real pain, and you will end up sticking fingers where you don't want them to be.
Good luck on your quest.
John