Custom Membership question

Hello,

 we have implemented custom membership provider, but we are having issue since the whole idea was to use same "User" database attached to the multiple different projects.

 To be more clear, if I create user on XX project, and use that user on the same project, all is working fine.

 But If I try to use same user on second SF project, it will throw an error on CheckValidPassword method, specific DecodePassword method (Bad Data error).

 var str = DecodePassword(original, passwordFormat);

 My question is, except DecodePassword, is there any other way to check if password is valid?

 Is there any limitation sharing one custom membership/database over multiple SF projects?

 Any other input would be appreciated.

 Thanks a lot

 Milos Tadic