Bass players may sometimes find themselves at the mercy of the drummers and guitar player's high stage volumes, leading many bassists to up their own onstage volume levels. If one amplifier can't do the trick, two certainly can. This is known as "daisy-chaining," where two separate amps and cab units can be linked together to allow the bass player to play at competing sound levels.
Place one amp on stage or in your rehearsal space where you have easy access to the controls. Place the next amp/cabinet near the first cabinet, no more than 2 to 3 feet away, to keep the sound even.
Insert the end of one instrument cable into the second amp's "Input" jack on the front of the amp's control facing. Plug the other end into the first amp's "Output" jack and plug in your bass as you normally would with a second, shielded instrument cable.
Plug each amp into a power source and adjust each amp's master volume prior to turning the amps on. Flip the switch or button to power up the second amp first, followed by the first amp. Adjust both volumes accordingly.
Make sure each amp has the same ohm rating (i.e., 4 ohms to 4 ohms, etc.) or you run the risk of damaging the amp or the speakers in the cabinets.