Cfm is a function of the pump output, not the storage vessel size. With more storage you'll be able to work longer the first time, but then it'll take longer to refill. Two compressors will theoretically fill twice as fast, but each is still only putting out what it is built to do. If you want more cfm, get a high cfm pump and motor adequate to drive it. With a 6 cfm pump you won't keep up with a sander, HVLP gun, or any of the other high volume users.

e.g. My compressor does 24 cfm @ 100psi. I can be using a d.a. sander, empty the 80 gallon tank, and it refills in less than 4 minutes. In real operation it doesn't empty the tank before it restarts the pump. The benefit of a high volume pump is, it will refill the tank WHILE I'm still using a high volume air hog like the d.a.