How to do multiple inheritance with sqlalchemy models

I would like to have a model class that inherits from 2 other model classes. Here is something that should work but it doesnt.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_ECHO'] = True
db = SQLAlchemy(app)

class BaseModel(db.Model):
    __abstract__ = True
    id = db.Column(db.Integer, primary_key=True)

class A(BaseModel):
    a_name = db.Column(db.String)
    a_type = db.Column(db.String)
    __mapper_args__ = {
        'polymorphic_on': a_type,
        'polymorphic_identity': 'a',
    }

class B(BaseModel):
    b_name = db.Column(db.String)
    b_type = db.Column(db.String)
    __mapper_args__ = {
        'polymorphic_identity': 'b',
        'polymorphic_on': b_type
    }

class Inheritance(A, B):
    a_id = db.Column(db.ForeignKey(A.id), primary_key=True)
    b_id = db.Column(db.ForeignKey(B.id), primary_key=True)
    name = db.Column(db.String)

db.create_all()
db.session.add_all((A(), B()))
db.session.commit()
db.session.add(Inheritance(a_id=1, b_id=1))
db.session.commit()

I got this error

sqlalchemy.exc.InvalidRequestError: Class <class '__main__.Inheritance'> has multiple mapped bases: [<class '__main__.A'>, <class '__main__.B'>]

What is wrong? I understand what the error says, but I dont know how to fix it.

Can anyone give me a minimal but fully working example of how to set it up?