Hi Roxburgh,
Your wife is free to choose which ever property she wishes to have treated as her main residence, provided the declaration is made in writing within two years. This does not need to be the same residence as you declare as your main residence.
CG64545 advises that "Where a notice is not made, or an invalid notice is made, the residence which attracts relief is the dwelling-house which is the main residence as a matter of fact".
(
CG64545 - Private residence relief: two or more residences: no valid notice made).
Thank you.