text: Title: Migrate to multiple databases simultaneously
  Body: Greetings. I am doing a project and I need to migrate to several databases simultaneously. For example, I have bind 2 databases in SQLALCHEMY_BINDS like that : 
       
       **app.config['SQLALCHEMY_BINDS'] = {
      'bobkov1': 'postgresql://postgres:zabil2012@localhost:5431/bobkov1',
      'bobkov' : 'postgresql://postgres:zabil2012@localhost:5431/bobkov'
  }**
  
  
  And now i want to migrate models to both of this databases. I'm tried to do it like that:
  
  **class User(BaseModel, db.Model):
      __tablename__ = 'user'
      __bind_key__ = {'bobkov','bobkov1'}
  
      id = db.Column(db.Integer, primary_key=True)
      username = db.Column(db.String(64), index=True, unique=True)
      email = db.Column(db.String(120), index=True, unique=True)
      password_hash = db.Column(db.String(128))
      posts = db.relationship('Post', backref='author', lazy='dynamic')
  
  class Post(db.Model):
      __tablename__ = 'post'
      __bind_key__ = {'bobkov','bobkov1'}
      id = db.Column(db.Integer, primary_key=True)
      body = db.Column(db.String(140))
      user_id = db.Column(db.Integer, db.ForeignKey('user.id'))**
  
  
  When trying to run this code, the model migrates only to the main database, which is defined in SQLALCHEMY_DATABASE_URI.
  
  Help me please, how to configure that?