We recommend turning up the volume level controls all the way on both subwoofers. Listen to a piece of music you are familiar with that has intensive basses.
Alternatively, you can also use the test sound of the receiver/amplifier. Now ask a second person to lower and raise the volume of the one subwoofer and then that of the other subwoofer. As a small aid, you can disconnect the normal speakers so that only the subwoofers are running. Both subwoofers are correctly adjusted when you have the feeling that both are equally loud.