Vid Luther has a new post today looking at connecting to multiple databases inside of your Symfony framework application. The key is in setting up your YAML files the right way.
Since you need a schema.yml or xml file, you can either generate one using a tool that will take your data defintion and create the YAML for you, or you can cheat and build a temp project, configure it to work with earlyprototype , and then do a symfony propel:build-schema . This will give you a schema.yml file in your config directory.
He walks you through the creation of two simple YAML files with database connection information and how to drop their contents over into the main database.yml file and some code to run the queries on the different databases.