Well, in theory, you would get twice the light from two bulbs. Reality is, I don't think you get quite all of it due to the close proximity to each other that the bulbs are mounted, but it should be about 99% of it I'll bet.
Other than the cost of the lamps, you will more lumes of light output. Also if you have a mirrored reflector it will help out. If its for a garage you want to get a 4100 color lamp that is a cool white color and it will brighten the work area up much better and cast less of a shadow.
I cannot tell you what the lumen difference is between 1 & 2 lamps, but from what I understand there is little difference between a 3 & 4 lamp fixture ,at least when discussing troffers which the OP is not,but perhaps consulting the fixture manufacturers photometric chart for the particular fixture in question could be helpful.
This site has links to large number of electrical related manufacturers: www.electric-find.com I use it when cannot remember the web address.