The second Snowflake bridge is now part of the stable release 12.0.
To use it, you just have to select Snowflake from the Built-in Bridges menu. How it works is: both the snowflake-01 and snowflake-02 bridges will be activated when you choose Snowflake under “Select a Built-In Bridge…”. You can tell which bridge is being used for a circuit (in the URL bar). If the IP address is 192.0.2.3, it is the snowflake-01 bridge. If the IP address is 192.0.2.4, it is the snowflake-02 bridge. The IP addresses are just placeholder labels (the browser is not really connecting to those IP addresses).