ADOBE DIGITAL EDITIONS - MERGE ACCOUNTS

Good Evening.

I have one account where i have buyed 1 Book (account EMAIL1)

i have another account (account EMAIL2) where i have buyed other 3-4 books (i forgot i have already buyed the book on another account)

how i merge both account in one ? actually i use one account on PC and other on IPAD ... but i fell bad to see some book on one device and some in another.. thanks in advance