The statistics are stored by account - not by device - so it's easy.
Just add the vivofit 2 to your account and then set it as the tracker for the account.
The trick though is to do it first thing in the morning because if you do it later in the day, the 2 will have to exceed the steps for the day that the 1 counted before it can add to your total.
What I do is wear both to sleep and, the next day, since both devices are counting since the moment I woke up, they should be almost have the same steps when you switch over. That way, you don't lose anything.
