When you have more than one disk in a Time Machine setup, it will cycle between all the existing disks in a round-robin fashion, and just add a new snapshot to the disk it selected (if it will fit). If it won't fit, TM will remove old snapshots until there is room to fit the new one. You don't need to erase anything except when you add a completely new disk to Time Machine setup (and perhaps not even then depending on the type of file system the volume was formatted as). The Time Machine keeps track of what disks it can cycle between to create the backups it needs to.
When you add a disk to the Time Machine setup on your system (by clicking on the "Add or Remove Backup Disk..." item in the Time Machine System Preferences pane), it asks you to select a disk from the list of available drives and then just adds it to the list of drives that TM will pick from during the next backup. It might ask you to erase the drive if it does not have a file system that TM can use.
If the drive is no longer mounted on the system, TM will just not bother using that disk, even though it remains in the list of drives in the TM setup in System Preferences.
So a couple of things to make absolutely sure of -- you cannot unmount a drive when TM is in the middle of backing stuff up to it and you can't unmount it if any other process is using it (like an open Terminal window that has "cd" into the TM drive to look at something). You can use Disk Utility or the diskutil Terminal command to unmount a TM volume and it will either just use other TM volumes that are mounted for the next backup or complain if there are no volumes available at all if you happen to unmount all the volumes that it knows about. If you try and unmount a disk and get an error message about it being "busy" then some application is in the middle of using it and you need to resolve that before being able to successfully dismount it.
Hope that helps you understand the process of cycling thru several TM volumes -- ask for more info if you need more details.
Good luck...
