Switch on both the guitar amp and bass amp and connect each instrument to its respective amp. Ensure the volume knobs on both instruments are turned up fully and any tone knobs are set to a neutral position, i.e. around the midway point on the dial.
Set the volume levels on the amps such that both instruments can be heard at an equal volume. Remember that a bass guitar is usually played one string at a time, so the volume of playing one string should be roughly equivalent to the volume of a guitar strum in order to establish a workable mix between the instruments.
Play one instrument at a time, and adjust the amp tone settings to produce the desired sound as you play. The objective is to find a tonal mix between bass, mid and treble that covers as full a range of EQ frequencies as you like, although the precise sound you set will be a subjective choice.
Play both instruments at the same time and listen to the mix of the instruments. The bass should be heard through strummed guitar, and the guitar sound should sound full and clear through the mix.
Adjust the tone knobs simultaneously while playing on each instrument to fine tune the sound and the blend of sound between the two amps.