メイン

app.py

from flask import Flask

from page1 import page1_decor
from page2 import page2_decor

app = Flask(__name__)

app.register_blueprint(page1_decor)
app.register_blueprint(page2_decor)

サブ

page1.py

from flask import Flask, Blueprint

page1_decor = Blueprint('page1_fn', __name__)

@page1_decor.route('/page1/')
def page1_fn():
#以下略

page2も同様