Re: 4 CFM + 4 CFM = 8 CFM?
It will work. As noted above, use check valves for safety. If one compressor fails, it prevents the other from pumping into it.
And yes, use seperate circuits.
But the trick is...if you really want to use tools that have a short high demand, just install a bigger tank. I bought the Husky compressor setup from Home Depot...32 Gal tank...small compressor but it also included impact wrenches, spray guns, etc...for $300. Works just fine for the impact tools....and short durations of sand blasting...but you wouldn't want to run an air sander off it for long....the compressor will never be able to keep up.