Lots to process here:
> So, I'm wondering if I can just reformat it as a regular drive
You mean two regular drives, right? Or are you trying to maintain a RAID 1 volume, just not using APFS?
> Then, in order to backup to it, I will also need to convert it to an APFS disk. Right?
Not necessarily. It depends on your backup method - since you already mention SuperDuper, that's your limiting factor.
Ultimately, though, I'd generally try to format the backup drive in the same way as the source drive you're backing up (Time Machine not withstanding).
> If so, I suppose the backup would go faster as the external drive has two disks in it
Umm, no. If you format the drives as standalone (no RAID), then you're only backing up to one drive at a time.
Even if you retain the RAID 1 array you will get no speed benefit - RAID 1 mirrors the data to multiple drives, so may actually be SLOWER than single disk writes since the data has to be written two times - once to each drive. Now, this should happen in parallel, so the time difference should be negligible, but it certainly won't be faster than a single-disk solution.
RAID 0 would be faster - data is striped across two drives, so each drive only has to do 50% of the work. But you get no data protection in this model (if either disk dies, so does all your backup data).
> After I backed up, would I then be able to use the APFS volume as a startup disk, if I chose to do so?
Again, that depends on your backup strategy - are you cloning your boot drive, or backing it up? Clones may be bootable. Regular (differential) backups are not.
> Now, I'm also wondering if it's possible to un-raid and format each disk separately as an APFS volume
If you mean two separate drives, that should be possible. It's kind of what I assumed you meant from the beginning.
> I'd have two APFS volumes in the same HD external container. Is that possible?
Assuming the enclosure presents two drives to the OS, their physical placement is irrelevant (one enclosure, two enclosures, bare drives sitting on the desk... it's all the same to the OS since it doesn't have eyes to see their physical placement :) )
> And if I did so, would I be able to reboot, (if I wanted to use either one as a startup volume), would I be able to startup from either volume?
Again, if one (or both) the drives is a clone then you should be able to use it as a drop-in replacement for your internal drive. If it's a backup (such as a Time Machine backup) then not (although you can use this through Mac OS's Recovery Disk option).
Ultimately you need to decide what it is you're protecting against. Honestly, the failure rates of modern drives is pretty low, so the chances of your internal drive failing to the point of needing to boot from a backup is pretty low, and most people care about the data more - even if that needs to be restored to some other device.
If you're worrying about some other hardware-level failure in your system then there may be some benefit in being able to boot up some other machine with a recent point-in-time state, but depending on your machine (and the failure state), it may be easier to just remove the internal drive and attach it to some other system. Lots of options.
