Two T5HOs should do just fine on a 75. That particular light, while affordable, isn't the greatest reflector or ballast wise, ergo I think you are looking good.
To explain further, I was looking to purchase the same fixture for my 90 gallon, I dose moderately and have Co2, and my plan was to pull one of the bulbs and run it like that (2 active bulbs)...exactly like you are.
Every tank is different...blah blah blah, but I think two of the bulbs running on that fixture, on that tank, with Co2, will work about perfect. As always, monitor and be responsible for your own situation.