Thats enough MORE than enough light for both plants, they should grow like mad in there. You might want to consider getting some kind of Hygrophilia in there, if you've got a lot of nutrients in a tank with halides, you'll have a ton of alage without something to suck up the nutrients. I'd throw some Glossostigma right under that halide, that'd be ownage.
not to burst your bubble, but metal halide bulbs only will sufficently light 1' in either dirrection, so for a 6' tank, you would need 3 bulbs. but you can always contain the plants to the section of the tank dirrectly under the bulb. but anyways, if the plants are under the bulb, they will flourish.
mh lights will light an area in proportion to the angles of the reflector, with a large reflector you can light a much larger area without loosing too much of the intensity, where i work we make mh lights to light up fish farms and believe me 2 400w bulbs can light up a 30' X30' pen easy