How to manage two ereaders

Hello, I'm sure there is an obvious answer to this, but it is not obvious to me. 

We have two Sony ereaders and two library cards.   Both library cards and ereaders are authorised and recognised by ADE but I can only download library books to one device.   If I download to the other ereader instead, not as well, I get the copy protected sign and cannot open the book.

If any has a solution I will be very grateful.