The Carl Martin Match Box solves the issues we face when using two instruments through the same amplifier.  The Match Box is basically an A/B Switch with independent Gain controls for the instrument inputs.  Plug two instruments into the 1 & 2 Inputs, connect the Output to either your pedal board or directly into your amp and adjust the Gain controls on the Match Box so that both instruments provide the same Output Gain to the amp or pedalboard.  This eliminates having to reset Gain levels on the amp when switching guitars, and allows the player to move between instruments quickly and seamlessly during their performance.  The Match Box works equally well for guitar (electric and electric/acoustics) and bass.