This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You can have as many as you would like. It will try them in order and if all have been tried a suitable exception will be thrown. You can provide multiple <security:authentication-provider> tags inside the <security:authentication-manager> namespace. Alternatively you can declare a bean of type org.springframework.security.providers.ProviderManager and set a list of providers on it.