how to use multi member ship provider in web config and on page

HI,
i want to switch membership provider if necessary after login button click.
i have added two in web config as below.

  <add name="AspNetSqlMembershipProviderOnLine" type="System.Web.Security.SqlMembershipProvider" connectionStringName="ApplicationServicesOnline"              enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" requiresUniqueEmail="false"              maxInvalidPasswordAttempts="5" minRequiredPasswordLength="6" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10"              applicationName="/" />         &nbs ...