Note on 2 of our 3 Macs Time Machine stopped doing any backups after adding a new drive to the pool. A restart fixes it.

hamarkus: the first hourly backup will go to the first drive, the second hour to the second drive, etc. i.e. Time Machine cycles through the drives each time it does an hourly backup.

Will the daily backups (after 24 h) also be distributed over the two backups, and the weekly backups as well? Or will both backups contain all daily (up to 30 days) and all weekly updates (which would make them close mirrors, not full mirrors as the backups will always be one hour apart)?

The latter: each volume Time Machine backs up to is treated as its own set. I have two drives which are permanently connected, so have almost identical backups on both of them (within an hour, as you say).