Two approaches: 1) Do what you did before Postfix 3.0.0. That stuff still works.
2) Build and install Postfix once; that is the first instance. Then use MULTI_INSTANCE_README to create the second Postfix instance. The two instances share the daemon_directory, command_directory, etc., but each instance has its own queue, configuration and data directories. If your distribution has broken the features that MULTI_INSTANCE_README depends on, - Manually add the second instance's configuration directory to the first instance's alternate_config_directories setting. Wietse