Alternate backups between two different external HD's

Hi,

Apologies if this has been answered previously.

I store my backup Hard Drive off site. This is pain when I want to back up because I have to retrieve the HD and then return it off-site after backing up.

Is it possible to alternate backups between two different HD's so I can backup to one HD and then take it off-site and bring back the other HD ready to be used for the next backup ?

Thanks. Mark