I don't think that it ever specifies the look of the district 13 people. I think they were miners and scientists originally. The only physical thing that I can remember is the yellow bits in one of their eyes.
Yeah- I hadn't really thought of who could play them. They would need to look like they had been trained for fighting. I guess I had thought someone like young Sigourney Weavers. But I would hate for them to either use one actress for both of them, or two actresses who are not related that they try to pull off as twins.
I was able to track down a link from the message board on IMDB for Once Upon a Time for the Spear Twins. They could actually be really good for the roles. They definitely look strong, and they look very similar. They do look quite make-uped up in that photo though. The Leegs should be a bit more plain.