You want a A/B/Y selector pedal - switch between two instruments or two amps. Useful tool!
Though it would be nice if Boss let you bridge the outputs A+B so you plug guitar into input A mono and bass into input B and have the option to bridge output A&B to one amp...
Yeti n other news, turns out looping is hard.
I'm waiting for a S.O.S from Norio... There IS a trick to it. You HAVE to play through the entire bar...think about a simple 8 count -> 1 and 2 and 3 and 4 and - where peeps loose it is that you end the LOOP when the count repeats at one - most rush and don't play the last and fully and end the LOOP too soon (or wait to hear the one and end the LOOP too late).
Practice with a drum loop and then when you are getting the drum loop repeating nicely (P.s. it'll never be 100% perfect) then try with guitar/bass/etc.