Possibly.
Compressors are rated a flow at a given pressure. If they both kick on at the same time, and are running together it will work as you describe. If the pressure switches are set different, you will still only flow the amount one compressor will put out. But at 40 PSI, both should be running, if not you need to adjust the pressure switches. You will have more CFM output than that when running on the tanks storage capacity (which will basically be dependent on what you are operating, the size and length of the air lines, and any restrictions you have in the system. You will only be down to 8.5 (17 with both running, when you are flowing enough air constantly that the compressors can only maintain 40 PSI. Likely they either be very small compressors, or you are flowing a TON of air. Unless you are using like a 1" impact wrench constantly, most things you would use in the shop you are on and off enough that the compressor can recover a bit. A die grinder used for long periods of tiem will flow quite a bit of air, but your hand will likely need a break before you are taxing two compressors.
FWIW