Bob's right on. More tanks only means you get more time to run the DA, but it takes an equal amount of time to fill the tank after it goes "flat." You get no net gain in work time.
what if I run two 5hp air compressors into a central tank. Will the cfm be higher? Would it then be equal to the two output cfms added together?
Yep. Compressors in parallel are additive - or at least theoretically. The flow not work out to be twice the as much as one, but it will certainly be more than you get from a single unit.

I've seen several shops with more than one compressor feeding a single line.

If that's all you have, I'd give it a shot and see what happens.