It depends on some factors such as what kind of mixers, what audio setup you have and what signals or instruments are being mixed.
Basically, if needed, you can cascade mixers by sending first mixer's *line* output (caution: not amplified output, if any) into one of second mixer's input channels.
I did that a few times, when using a Roland keyboard which doesn't have separate knobs for headphone and line gains - used a small 8-line Makie mixer for the headphone / output control, in series with another 24-channel Makie mixer for the main mix, and did the job in the live environment.