I remember having this particular conversation with myself a couple of years ago and well and truly over-thinking things to the extent that I went out with two playouts including two mixers and an electronic gizmo that combined the signals from both desks to the PA. I also had a USB combiner so that both laptops could be connected to the mixer at the same time. Twas a pain in the backside to set up every night.
So....what do I do now?
My main playout is a MacBook (it's just my preference, it used to be a PC but I went to the dark side a year ago and didn't look back). That's connected to the MC6000mk2 and out to the PA...simples. However, next to the main playout is a SECOND MacBook (I'm either incredibly rich, stupid or both) which controls the lighting (I don't like to mix the two). BOTH MacBooks are identical in that they have the music libraries, the lighting control and VDJ on them (so technically I can run the whole shebang from one if I wish to or if something goes wrong with one of them).
The second MacBook isn't actually connected to any sort of mixer or the PA, because my next action depends on what goes wrong.
Should the main playout machine go down, it's a quick swapper of the USB cable to the Denon (and a little banter on the mic) and the show goes on. If the Denon goes down, I have a DJ2GO2 sitting on the booth which can be quickly swapped in (again with a little banter to save dead air for the 5 seconds it takes).
If both Macs go down, I have an iPad and an iPhone with emergency libraries on them which can either connect via the Denon OR use the magic of Bluetooth to connect into at least one of the Maui's if they're the PA of choice on the night and depending on how I've connected them. With both of these, I use Djay2 which although isn't ideal....is very good on "auto mix" which will get me through any immediate troubleshooting.
Yes, I have the risk of a little "dead air" should something go wrong until I swing whichever backup into place....but at least it's only a couple of seconds and the party will continue. There's a LOT of DJs in this area that aren't anything like that resilient
Sorry....I ran on a little bit there