Mary Stuart Masterson and her husband, Jeremy Davidson, are expecting twin babies this summer.   Masterson,44, and Davidson,39, already had a 2 year-old son together.   The stars join a large brood of celebrities that have added twins to their families in recent years.   Mariah Carey and Nick Cannon are also expecting twins: a baby boy and baby girl this summer.  We do not know the gender of the Masterson twins yet.