If you've got even two 5.0 UVB bulb located more than 12" from your uro, you'd better be supplementing with D3. All of the tests I've seen show that the UVB output diminishes exponentially as the distance increases. In other words, at say 14" you may not be getting half of the usable output as you would at 12" and at a distance of 18" from you're uro, you're clearly wasting your money.
Doug Dix also told me you can't always believe what the manufacturers tell you about their bulbs. Some bulbs don't have the advertised output, or drop off substantially in less than 6 months. He demonstrated this to me with a few simple measurements with his UVB meter. It was very eye opening, but since I've got an engineering backround, not terribly surprising.
I'm following what Doug does, since he's been at it for a long time, tests his bulbs and has personally tried and trued his setups.
Craigo