If the subs are located close to each other, the gain will be almost 6dB, that is, 2x the amplitude. If they are far apart, gains usually range from 3-4dB.
Also, if the user is crossing to speaker with limited LF output around the range of 70 Hz and above, it is greatly beneficial to locate a sub near each main channel, specifically, within 1/2 the crossover frequency wavelength distance. This ensures that each main speaker acts as a virtual single source at and around the crossover frequency with the subwoofers, resulting in seamless bass for music reproduction. HT only application does not usually need this level of integration. It is not as sound quality critical as music in most cases.
-Chris