I agree. Lumenarc's are great for light spread, and they spread that light in a square pattern. 2 large lumenarc III or large lumenmax w/ 400w bulbs would be good for a 6'x3' footprint, ie. 3ft/sq for each large square reflector. Lumenarc mini and the smaller lumenmax are good for 2ft/sq or on a 6'x2' tank I would recommend 3 x 250w. Since they are lighting a smaller footprint you can drop down to 250w.
In either case there is no way you are going to get 2 square reflectors to light a 3'x1.5' rectangle evenly. 3 reflectors in the best choice and still isnt perfect but will probably look the best.
Of course if you arent too concerned with how even the light is you can do what ever you want.