Sorry, I amend that last. Titles may be relevant according to what you know of Lord and Lady Whotsit and how intimate you are with them. Professional qualifications are for the workplace only.
Thanks to everyone for your replies.
I do call them by their first names when writing the inside of their card but she's worked so hard to get her doctorate that I thought it would be nice to address her by her title on the envelope.
Ta again x
It shows courtesy by acknowledging the professionally qualified before the rest. Therefore, you should address the envelope Dr FirstName LastName and Mr FirstName LastName