Where my work is, the nearest emergency 24 hour clinic is 20-30ish minutes away, which isn't too bad. We accept emergencies 24 hours a day, 7 days a week, and have a dedicated emergency phone the doctor has on her all the time. We (receptionists, assistants, and techs) each picked up a day of the week to be on call overnight. With having 6 months in at this place, I've never been called in, but the doctor gets calls regularly. The doctor usually handles the emergencies on her own if she can or determines if it can wait until the morning. There's only 10 of us, including the doctor and manager. We are pretty much all there 5-7 days a week.
That's why I'm thinking it might just be my experience in that I wouldn't be comfortable getting a second job with set/inflexible hours. Two girls I work with have second jobs at a pizza place, but they are able to come and go as they please so it works. I was hoping to get a part time job at a grocery store so I could use the discount for my parents, but I can't ever guarantee my hours.