The drive in use situation wouldn't be normal and I don't believe it would be due to OneCare, though I could be wrong.
However, once you connect a different drive (you mention selecting the 2nd identical drive) you must reconfigure backup. OneCare backup is not designed for rotating destination devices out like I think you are wanting to do. It is simplistic in that it expects the drive to be connected to the PC when backup runs.
-steve