use 'bind_key' to specify which database to use in models

__bind_key__ = 'X' in a model will point to the database associated with key 'X' in the config

    SQLALCHEMY_X_DATABASE_URI = "connection string"
    SQLALCHEMY_BINDS = {
    'X': SQLALCHEMY_X_DATABASE_URI,
    'Y': SQLALCHEMY_Y_DATABASE_URI
    }