If you are contemplating a 2 yr separation, no fault divorce, and you are both 100% in agreement, then you can start the separation clock whenever you wish. It is between the two of you and the judge who will hear your nisi.
If one or other of you isn't happy about divorcing, then as Dukey says, you have to live separate lives for two years - No cooking, washing, sleeping, socializing with each other.
Yes, that was part of my reason for asking. I've spoken to my wife stating that if she wants to get divorced she will need to instigate it and be able to give reasons as she is the unhappier out of the two of us.