But I would still like for all the devices to share a single iTunes account, so that
- I only buy an app once
- I want to be able to log into Find my iPhone with my main iTunes account and still see all my other devices.
One iCloud account is the primary account and only that account can use Find My Phone and Backup services, so to have one account with multiple devices implies that that same account also gets the iCloud backups.
However, iCloud doesn't co-mingle a backup from one iPhone 5 and another iPhone 5 (although you may want to choose different names for each to avoid confusion when restoring if you happen to have two of the same device).
Unless you are OK switching up accounts manually to push a backup to an alternate account and then switch right back, there currently isn't a way to do this with stock / unjailbroken iOS.