How Can We Have two Connection Strings In Web.Config And Switch Betweeen Them In Code Behind?

When you add a connection string, you name it.

You can access each such connection string and assign it to a different variable, passing that connection string to your data access layer.

In the config file:

<connectionStrings>

  <add name="Sales" 
       providerName="System.Data.SqlClient"
       connectionString= "server=myserver;database=Products;uid=<user name>;pwd=<secure password>" />

  <add name="NorthWind" 
       providerName="System.Data.SqlClient" 
       connectionString="server=.;database=NorthWind;Integrated Security=SSPI" />

</connectionStrings>

In your code:

 var conn1 = ConfigurationManager.ConnectionStrings["Sales"].ConnectionString;
 var conn2 = ConfigurationManager.ConnectionStrings["NorthWind"].ConnectionString;

Simply put those strings in your web.config:

<connectionStrings>
    <add name="CS1"
         connectionString="SOME CONNECTION STRING"
         providerName="System.Data.SqlClient" />
    <add name="CS2"
         connectionString="SOME OTHER STRING"
         providerName="System.Data.SqlClient" />
</connectionStrings>

And then pick the one you wish in your code:

string cs = ConfigurationManager.ConnectionStrings["CS2"].ConnectionString;