require n out of m keys/users to authenticate an ssh session?

[laghao]

I'm looking for an option - eventually to extend standard ssh - in such a way that I need (at least) two people/keys out of m possible to authenticate a session instead of one out of m known once...

anyone seen this or know how to do?

I know there is key + password (which is kind of this direction) but not exactly what I'm looking for...