No, not if they're written by the same hardware using the same process. You need to restore data from one of them, and then another one every so often. Not every two hours, though.

Maybe, it depends on your data-loss and time to restore flexibility. Test regularly enough that you are confident it works. If you don't test at all you can't be confident that it works at all.

For instance, if you have a full/diff/log backup cycle for a database for instance, perhaps test restore each full backup. Just be aware that if a full backup happens daily your comfort zone for data loss in the case of a disaster needs to be at least a day worth of work.

Also, if you backup a range of data and your method allows for partial restores, you might do partial restores of key information far more often than you test everything. You could spread the testing load temporaly: check the really import parts every time and cycle through the other bits less often.

Also if your concern is man-time, automate the process as much as possible. My mail server (running Zimbra) has a small replica in a VM that restores itself from the latest backup once a day and sends me a message to say what the last message in its queue was. This way if I don't get the message, or it says the last message was too far ago, I know something has gone wrong and the backup or the restore failed. I manually log in regularly to inspect a little deeper too, this is slow as the VM has far lower resources than the main box. If properly separated from other resources (so there are no single points of failure that can take it out along with everything else - mine isn't currently that well arranged) then this copy becomes an extra secondary single-snapshot backup itself.

I really meant to restore in order to test your backup process. It's important to test your backups periodically to ensure your process wasn't inadvertently broken, but you don't need to test each and every backup.

No, not ALL backups, just the first backup you do: you test that you can restore the data from it.

Then you can be confident that following backups will be restorable.

Corrolary: If you introduce a new folder for another service in your backup, make sure you can restore it too.