I always use doctrine
If you are connecting to multiple databases
1. Specify the connection of the table when defining the schema

mytable:
  connection: conn1
  tableName: mytable
  columns:
    id:
      type: integer(8)
      ....

Define database connection parameters in 2.databases.yml

conn1:
    class: sfDoctrineDatabase
    param:
      dsn:      mysql:host=127.0.0.1;dbname=mydb;port=3306
      username: root
      password: 
      encoding: utf8 

I tested it myself and posted it first

2. It is best to add prefixes to tables in multiple databases to avoid having the same table names and causing some models to be overwritten and lost

3. Generate schema and yml files symfony propel:build-schema

4. Generate form symfony propel:build-form --connection=testa symfony propel:build-form --connection=justb

5Generate filter symfony propel:build-filters --connection=testa symfony propel:build-filters --connection=justb

6. $c = new Criteria(); $a = APeer::doSelect($c, Propel::getConnection('testa')); $b = CPeer::doSelect($c, Propel::getConnection('justb'));