A single AdobeID can handle up to 6 Desktops (Windows or Mac) and 6 attached devices (e.g. EInk Readers). You can also download lending library ebooks in the Adobe format (PDF or ePub). However, this does not narrow the field much, since almost all EInk readers except the Kindle support Adobe Digital Editions.
At about $200 per device, I think your best best is the Astak EZ Reader Pocket Pro. This has a 5" screen. You could also get one 5" Pocket Pro and one 6" EZ Reader Plus (the upgraded version of the EZ Reader Basic). These are very similar devices, except for the screen size.
The 5" Sony PRS-300 is a very basic reader and has a relatively old version of Adobe Digital Editions, but it works well enough and is widely available. Don't forget to include a protective cover (the EZ Reader's come with a cover).
For other options, see
E-book Reader Matrix.