Excellent article on kempie's link, except for this bit: "The best way to ensure that HMRC will recognise the property as your PPR is to formally nominate it as such by writing to your tax office to confirm the position. If you have more than one property you should ideally notify the tax authorities as to which is your principal residence for this relief."
You can tell HMRC which of 2 residences is the principal one but if you own 2 properties and only live in one, then that one is your only residence and must therfeore be considered your principal residence.