I'd go with one speed controller for each motor, unless you want them both to do the same thing (i.e. both backing, both ahead). It would be more realistic with two.

As for Jerni21, the gas engine sounds like a cool idea, but there are two problems that I can see: what will you use for the gearing, and how are you gonna make it back (go astern)? and unless you put two engines in her like the real thing, both props are gonna go ahead or astern at the same speed, and without flanking rudders, it's going to be tough to maneuver her back. Also if you do that, think about your props, they'll both have to be right hand turning, or both left hand turning wheels. Or you could go with one prop, and a nozzle. Just some thoughts...
KB