Not quite sure what the question is getting at. I have one account. I can add amounts to any of the available fixed terms whenever I like. They are then listed on my account as separate holdings with start and end dates. Thus I have several three, four and five year holdings of various ages from 2014 (but nothing since the several drops in rate* offered - they have to pay for their TV adverts somehow). Whether you can have two accounts I know not.
My eighteen month tranches are at 4.65% and 6.5% (special promotion March 2015) Three year at 6.5% (now 4.8%) Four year at 7.0% (now 5.3%) Five year at 7.5% (recently dropped to 5.8%)