You might try two instances of Thunderbird - one for you and the other for your wife - i.e. Two completely separate accounts.
Sharing an e-mail account is rather like you both keeping your underwear in the same draw - it can lead to confusion.
Imagine that you're not man and wife - just two independent people sharing the same computer - would you want your house mate to be reading your Private Messages?
Good luck..., November 2014