PAR isn't something that you put into the tank. It is the intensity of the light at various points in the tank. The light can come from room light and/or one, two, three, etc. lights sitting on top of, or hanging above the tank. When you have two lights close together, their light overlaps a lot, so the intensity can double right under the bulbs. But, if the two lights are widely spaced, say 8 inches apart, the overlap is mostly of the lower intensity area away from directly under the bulb, so the intensity in the middle of the tank is more likely to be about the same as when only one light is over that spot.
A 90 gallon tank is about 18 inches front to back, so it is helpful to have two lights, widely separated, to get more uniform light intensity over the whole substrate. If you put two light close together you will get possibly double the intensity right under the lights, but near the front and back of the tank the intensity will likely be much lower.
If you are sure you want high light, you can use a Catalina Aquarium 4 bulb T5HO, with the bulbs in two groups of 2 bulbs, with the groups about 8-10 inches apart, in an extra wide housing. That will be a custom light, but they have made some like that for other people.