According to the PAR vs Distance for Various T5 Light Fixtures chart on the first page, the AquaticLife T5HO 36" 4 bulb fixture has about 90 PAR at 25 inches.
Is it safe to assume that the same fixture with 2 bulbs has about 45 PAR at 25 inches?
I have glass covers on top of my tank and they build up quite a bit of condensation.
How much will the glass/condensation on the covers reduce the PAR of a light?
That is a safe assumption, that you would get half the PAR with half the number of bulbs.
The condensation on the underside of the cover glass should not appreciably reduce the PAR, but the hard water deposits it may leave behind will reduce the PAR. You need to keep the glass as clean as you can if you want to get the full PAR from the light.