theoretically, for a short period of time your cfm capability will be higher. (for instance, if you dumped both tanks over a period of a minute, you would have had about 30 cfm, but the pressure wouldn't be consistent).
In reality you couldn't get that much through wimpy little 3/8 air lines, but you could run tons of air for the short period of time that the tanks will be charged. . . .then you rely on the motor.