I have a very big app written in Yii1.1. Is there a way to update it to Yii2?
It is a complex app which consists many database tables and uses many external services for inserting data to the app via tablets submit forms.
Also if it is not possible can these 2 Yii versions collaborate in a single app?
Automatically — no. Manually by partial rewriting — yes.
See Special Topics: Working with Third-Party Code | The Definitive Guide to Yii 2.0 | Yii PHP Framework
At first thank you for your answer!
partial rewriting the app in order to work with the yii2 features?
Yes. The amount of rewrites may vary. For projects where you directly use framework features without any extra isolation layers it might result in nearly full rewrite. For projects with a good separation of business logic it might be easy migration.