CJHL1947

Hello, I have a few questions for the new tax year that I am a little unsure of, If I have an employee who has 2 jobs, their first job being with another company & their second job with me. If they go over the NI threshold with their first job (not with me) would we then also pay NI even though they have not gone over the NI threshold with us? Is national insurance accumulative like tax? If I have a casual worker who works 40hrs one week and 2 hours the next, is the NI adjusted per wages every week? Thanks