@eschwartz: Is it possible for Calibre to do these things:
1) Keep track/info of books that were sent to device (like the built-in column "on device" that is visible when device is connected but not when there is no device connected)
in custom column . Ideally on per-device (2 Kindles) basis

So
2) Auto-magically update that custom column values based on "on device" column's data every time you connect the device ?
So as to have always visible track if you've already transferred to your device some particular book(s) and/or filter the library based on that and "read" custom column being "false"/"undefined" to get "have it on the device but didn't read it jet" virtual library.