I have only one line, which my S8 is on. But I do keep a Galaxy S6 in my car, and I keep the software maintained on it. I'm out of town for my work a lot, in a new place each job (so being able to navigate is very important). And with lots of points of contacts, so communication required. So if my S8 were to mess up on me for whatever reason, I can pop out the SIM from it and put it in the S6 and keep going. I keep the SIM card tool taped to the inside of the case of the S6. I'm all about redundancy.