Yes, you can have two (or more) backup drives.
- In the WHS console, on the Server Storage tab, right click the backup drive and select Remove.
- Ensure that Temporarily remove it from my home server is selected, and click OK.
- Disconnect the drive and take it off site.
- When you reconnect the drive it will automatically be detected and added as a backup drive again.
You can have more than one backup drive registered with the server, and alternate them using the above steps.