With both operating at all times, engine vacuum is reduced by about 29% in each (1 ÷ 2^.5 = .707). This may cause a power system to open earlier than desired, killing mileage.
Exactly why I have been suggesting carburetors with MECHANICAL power circuits (like the Carter W-1) for decades.
From the article referenced in the link a few posts above:
"The second criteria for selecting carburetors is that the EASIEST AND LEAST EXPENSIVE good results will be obtained by selecting either (A) carburetors which were originally designed for used in a multicarb environment (example: Carter WA-1’s used on the Twin-H Hudson) or (B) carburetors with a mechanical (as opposed to vacuum) power system. Carburetors with a vacuum power system may be used, but the tuning MAY be more difficult. If you have a spring winding kit and feel comfortable hand winding small precision springs, you may ignore this paragraph."
Jon.
