It happens you forget... You´re human
Here, three devices are synced to the same Calibre: A Nook Color (1st ed), a Papyre 6.2 (AKA Bq Avant) and a Sony PRS T1. Each is synced to different things, for instance magazines in color to the Nook, sci fi books to the T1, and tech books to the Papyre. I use the tags to know what content goes where, and sometimes one book can go to two devices at once. Edit your tags wisely.