There's nothing wrong with carrying a prepaid phone on a different carrier for improved possibility of getting a call through. Like anything else, it's a cost/benefit analysis that I can't help you make unless I know where you're going and what the coverage is supposed to be like there.

Calls to 911 calls may go through when your phone reads "no signal" if a competing carrier is available -- in the US the law requires all carriers to accept 911 calls even if a phone isn't activated. On the minus side, different carriers now use different incompatible technologies, so "no signal" really can mean that you're out of luck.

Texting often will get through when voice doesn't. This has worked for me countless times. In an emergency it's certainly worth trying.

I carry an iPhone. They're supposed to fall back to EDGE service when 3G isn't available, something I've used successfully many times. Since 3G and EDGE are the data services, neither is supposed to have an effect on voice availability.

There were two times, before I carried a cellphone, that I tried to use a Ham radio HT to communicate in an emergency. Neither time was I successful. That's not to say that it doesn't work when repeaters are available and people are listening, but when you consider the time, effort, and money required to get a Ham license and an HT, I'd sooner recommend a PLB.