I am also scratch building a commercial landing craft/ferry. The Henry Island ferry runs among the San Juan Islands in Washington State, carrying fuel trucks, utility trucks and all sorts of things. It is twin screw, so my model will be, too.
A twin screw boat needs to have opposite handed props, for the reasons stated above. I call this side thrust "prop walk" since it resembles a tire on the road-bottom of prop or tire moving left, side thrust is to the right. That side thrust moves the stern of the boat, more so in reverse. With props that turn outward going ahead you can use the prop walk to help spin the boat on its own length going ahead on one engine and astern on the other. This can be done with "tank steering," with the port motor on the left stick and the starboard motor on the right stick of your TX. Or with a motor mixer, such as the Action P40.