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gwern comments on Open Thread, September 1-15, 2012 - Less Wrong
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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 bit painful. Depending on how much accuracy you want, you could make up your own simple recurrence (imagine a list of 5 integers, which shift each day, and the first is defined by the sum of the last two modulo 5). But it turns out geeks have already discussed PRNGs you can do with mental arithmetic:
For the looks of them, at least one suggestion should work for you.
This allows pair members to authenticate themselves to each other, but not third parties to tell members apart.
Set up another pair of RNGs; both write down on a piece of paper and show the paper simultaneously, something like that. With third parties, you lose the time-delay aspect which makes things hard in the case of temporally separate pair members trying to authenticate to each other.