Can I borrow an ebook on one device and read it on another device?
This answer is applicable to borrowable eBooks available through Scholars Portal Books.
Yes, however; Adobe Digital Editions (ADE) is not a cloud-storage tool; downloading and opening a book on your laptop will not automatically add it to your iPad's reader, even though they are both associated with the same AdobeID.
If you've opened a book onto one device and wish to use it on another, you will need to locate the .acsm file you initially downloaded. Email the file to yourself or save it to Dropbox. Then open the .acsm file on your second device. As long as your second reader is associated with the same AdobeID as the first device, your loan period will continue on both devices.
(It may be faster to return the book on your first device then borrow it out with your second one, but there is a chance that someone will check out the book in the intervening few minutes.)