two bulbs will still "hit it" but that doesn't mean there is less light hitting it. you should have one T5HO every 3-5 inches of tank width. that beign said on a 12" wide 55g aquarium you'd want to have a 4x54watt T5 light fixture with individual lamp reflectors on a 90g that is 18" wide 6x54watts.
with 1 lamp per 3 inches you can hows monties on the sand bed of a 24" deep aquarium and acros in the mid.
T5HOs are blinding bright dispite what some may think. they over power most MH
lighting systems if done right and have a reflector that directs the light into a more focus beam penetrating extremely deep