On this I must disagree with @Pancho75. While you could get enough light out of one fixture, the coverage you will get in the tank will not be very good. You can generally figure that one AI Hydra 52 will be able to cover about 24 inches of tank length. Since a RSM 250 is about 25 inches long, you will be left with the ends of the tank looking very dim, especially near the surface.
I recommend installing both fixtures on your tank, but running them at about 25% of max to start. Later on you can ramp that up or down as needed. This way you'll get a much more evenly illuminated tank. Also, you will be able to really ramp up the blue light if you so desire. You will also be able to reduce the red and/or green light which can sometime help with unwanted algae growth.
Do keep a close eye on your corals initially, since it's possible to "burn" them due to the increase in light over the standard fixture/ Don't ramp up the light too fast.