It's possible to run them together and get 10.3 CFM. It's done all the time in industrial applications. Normally you'd stage them so that one runs for most of the light loads and the other one starts when the pressure drops further. This requires resetting the pressure controls, something that may not be possible on some of the newer smaller compressors. You should also plumb them with check valves so one can't feed into the other tank. If you simply connect the hoses together the regulators will likely fight each other, the one with slightly hihger pressure will force the other one closed. Even still 10.3 CFM probably isn't enough to run a decent sandblaster although Harbor Freight may claim theirs will run on less.
