意思不同.
in the meantime是在此期间的意思,
in the mean time是同时.
eg：In the meantime,the same problems that forced Beijing's hand in 2005 have returned to haunt Chinese policymakers a second time.
与此同时,曾在2005年迫使北京采取行动的相同问题已重新浮现,第二次困扰中国的政策制定者.
In the mean time,why not live each day as your birthday?
同时,为什么不把每天过的像你的生日一样呢?.