Seems like you're on the right path; you're ready to have a backup machine and just want to figure out what are appropriate choices, right?
And terminology is in flux these days. Lots of devices are both mixers and recorders.
Me, I've only had to reach for my backup mixer/recorder a couple times due to equipment failures. Both were stressful, but the producers were super happy that I was able to get back up and running in a few minutes (felt like hours for me). In those instances, my backups were my previous-generation equipment (audio quality the same, but fewer features and tracks). So I haven't bought two new mixer/recorders at the same time.
As for what products to buy, think about how many channels you need to record even in a backup/emergency situation, how portable you want to be (like do you want a "big" recorder/mixer for some jobs and a "small" one for others with one or the other always available as a backup)? And how much can you afford to spend?
With a 552, would you be OK with the non-timecode two tracks it can record internally (IIRC), or would you want to pair it with a 744T which would give you a bulky (by today's standards) but functional setup...though without a screen. Or would a Zoom F4/F8 be a better backup for a nicer Sound Devices or Zaxcom mixer/recorder? You'd have a couple different interfaces to learn (and you need to be able to work your backup recorder at a moment's notice), but that could work I think. Note that buying used can be a good thing, especially right now as Sound Devices and Zaxcom roll out new products...their slightly-older products (Which work just great) are showing up used at good prices.
So before we get specfic, what type of work do you do or plan to do? How many tracks do you need? Do you expect to be mostly on a cart, mostly working out of bags? What sort of budget do you have? And how much experience do you have? Buying both a 688 and a 633 to start out would be great, but that's a very serious expenditure. Anyway, let us know and people here can probably make some really helpful suggestions...