A mixer. You can either use the PA mixer, for example by running two DI boxes and using two PA desk channels; or you can buy a small mixer and put it on your pedalboard or in your rack. There are some mixers that are as small as an fx pedal.
Caution: only use active mixers, do not get suckered into buying a passive mixer. Passive ones are very cheap, so the temptation is strong, but forget it--they cause more problems than they solve.
