I was on DD and ended up having to call Eircom to shut the account & get a refund cheque (and stop getting their snam off them) months after the switch tookplace (Allegedly on the basis that I had Eircom Chargecards, which I thanfully was lucky enough not to use, but that's another story).
But all of this (how many accounts are open and for how long) is a non-issue as long as you take the time to set up DDs for both telephone accounts (preferably through a CC with moneyback)