testcase.py

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///"

db1 = SQLAlchemy()
db2 = SQLAlchemy()

class Model1(db1.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(256))

# db1.init_app(app)
db2.init_app(app)

app.route("/")
def index():
    models = Model1.query.all()
    return ", ".join(m.name for m in models)