Depends on the player.... I use a system called "tvheadend", which let's me assign multiple streams to one channel, I can set the priority it tries it in, how long before it moves on, etc... So that's great.

What's not great is it took about 5 days, and multiple hours over those days, to make it a cohesive setup..... There's probably a better option if all you want is channel fallback, but worth mentioning if you don't find anything. Happy streaming.