For two plants, I am going to assume you are after a low-profile grow.
#1: grow a pure indica strain because it is going to flower a lot earlier and it won't get too monstrous.
I would use a 150W CMH (Philips Ceramic Metal Halide) lamp hung vertical, no reflector, between the plants. Prune judiciously to keep leaves a minimum distance from the lamp. Line the walls with Mylar film for maximum efficiency. The CMH runs a lot cooler than the HPS and radiates most of its light sideways. Far nicer in every respect. I'm using one in my GH to help control veg time, because it looks a hell of a lot more natural than a bright yellow HPS!