好像听说有什莫条件。

sure, everything comes with a "if.."

如果2人都有401K能开吗？如果一方有，另一方没有能开吗？这部分钱免税吗？

If you and your other half totally earn 15w+, you cannot open Roth IRA.

Not deductable, but growth is tax-free.

Do a Google, there are plenty of answers everywhere.

e.g. finance.yahoo.com, www.vanguard.com

Your concerns are under the "retirement" topics.