Ok, so rotating two disks with Time Machine is rather simple. Probably simpler than using 3rd part to do backups (from this worth reading FAQ about TM) of it.
I actually like to keep half my macbook's disk for TM and I want to add an external disk.
Problem is: I can only choose one to backup on TM's settings.
One possible solution is setting up TM to a RAID, but I'm not sure this is a good idea, and I believe it would require both disks (or partitions) being the same size.
Currently I think it's better to just manually rotate, by going to settings and changing the selected disk daily, or whatever frequency.
I think the question here is: how can I do this automatically?