Spareparts is right about not starting both compressors at the same time. Running current stamped on your compressor motor is far lower than starting current. Locked-load current for two motors at the same time on home wiring would cause a big voltage drop.
The other thing that you want to consider is that the two compressors should not be mismatched to any great extent. Every component in a compressor has a max pressure rating. My compressor runs between 150 and a high of 180psi. I'd be a little leary of pushing 180psi down the throat of one rated at, say 140psi. Can the pressure relief valve or the unloader valve handle the pressure from the larger compressor? Is the tank rated at the higher pressure?
