.Net test multiple connections for AD authentication using system.web.security (membership)

Most of our .net IIS webapps use the System.Web.Security in .net to validate user logins in AD. However, during a company transition period, we currently have 4 AD domain controllers. I am looking for a way to test the AD user login against other AD connection strings in the event that one is down or that a user is using a different domain password.

Our code is just:

bool isValidAdUser = Membership.ValidateUser(model.UserName, model.Password);

And the connection string is set inside the Membership tag in web.config. How I can I add more connection strings?