Do I need to Split the Title to my Property

I have just built (with full planning permission) a second house on my land.  My mortgage (in joint names) is solely on the original house.  At some stage I will want to sell the second house but I may chose to live in it myself for a while.  At what point do I need to split the title?  
Many thanks
Howard Blakebrough

Howard - the split normally happens naturally when you sell whichever part - it's called a transfer of part