UserInfo.Username or UserInfo.Membership.Username for Username
UserInfo.Membership.Password for Password
in your codebehind should get you your desired result if you are looking to get the current users information there are other methods but if you are just looking for that information through code there it is.
Another method for logging into another site would be:
With DotNetNuke.Entities.Users.UserController.GetCurrentUserInfo
LoginToTheOtherSite(.Username, .Membership.Password)
End With
Hope this helps,
Michael