Rotate Passport Essential SE 1TB drives for Offiste storage?

I have two Passport essential drives which I rotate monthly. Will the SmartWare work with the two drives?  Can I just connect the drive or do I need to delete the previous backup files each time? 

Right now it doesn’t seem to recognize that it is a continuation of a previous backup and starts adding an entire backup to the drive I just connected.

The drives will be detected as two separate drives, not certain if this setup will work.

Is there any particular reason why it needs to be done like this?

The idea is to keep one copy off site, while the other is attched to the PC performing daily back up.  At the end of the month, I eject the ective drive, take the drive that was offsite and attach it.  In this way - the worst case scenario is I lose 30 days of files in case of disaster.  What I would like is that when I attach the drive that was odffsite, it just needs to do an incremental backup of that months files, but what I have been doing is deleting the previous backup and have it run a full backup.  If there is a better/smarter way then I’m certainly open to it.