Per person, but most people in ordinary day-to-day life will have plenty of opportunity to observe and practice mimicking their cohabitor's handwriting if they feel like working on that - they can't talk to each other directly, so they leave notes ("watch out for our left foot, it's still tender, I dropped something on it", "so how are you doing, what are you up to", "we're pregnant").
So handwriting is secure between a pair; then all you need is some sort of authentication. Why not use a very simple random number generator? Each member of a pair knows it, of course, and they occasionally set up fresh seeds. Each day is one iteration. To 'sign' a message, one simply writes down today's random number afterwards. (You said handwriting is secure, so you don't worry about someone tampering with the message and making an authentic number testify to a faked message.)
What RNG? Dunno. Blum Blum Shub has a hilarious name, but the multiplying is a...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.