@Shopaholic I was a bit confused too
Thanks for all the advise I think I'm going to keep them on the one account, Dad will just have to get used to logging in with Mums email address.
At the moment I periodically strip the DRM from mums books and back them up using calibre. Although she understands the function of calibre she finds it a little bewildering to use so I'm trying to keep it simple for them when I'm not around to help.