How do I confirm that each of two attached Time Capsules is indeed keeping a backup?
I love having two destinations and the best way to check is to force two backups, rotating between them each. You’ll know if the file system are clean, the metadata correct and be able to see the actual timing needed to back up to each. 

tmutil startbackup --rotation --block &amp;&amp; tmutil startbackup --rotation --block


You will get timing on the backup and can monitor the status in another shell

tmutil status


An alternate method would be to get a tool designed to parse the backup manifest like BackupLoupe

Focusing on the bands which are a binary representation of the file system seems like a lot of work when you can just mount the file system and look at the files stored. Each backup interval has a plist file showing what happened on that backup interval.

Side answers to side questions:


no the format for sparse bundles isn’t changing - when they fail to mount, you are likely going to either lose the data or wait a very long time and find out you can't write data to that destination again. By long time, I mean a week or more in some cases to repair or check a sparse disk on TimeCapsule due to the slow processor and massive IO count to perform filesystem checks of Time Machine hard links. 
When you have multiple destinations, the default behavior is to try for round robin, so with two connected destinations, odd hours would go to one destination and even hours to the other. So if you never sleep, instead of 24 intervals going to one destination (default case when you have only one drive) - you would get 12 backups to each of the two. If the machine sleeps, it should track roughly odd/even and the only imbalance happens when one of the two isn’t connected, the system prefers the “correct” destination, but will back up sequential intervals to the remaining destination if it’s the only one that mounts when requested.