is it permissible for Ḥanafīs to follow Shāfiʿīs in Witr Ṣalāh and vice versa?

It is permissible for Ḥanafīs to follow Shāfiʿīs in Witr Ṣalāh just as it is permissible for Shāfiʿīs to follow Ḥanafīs, provided a few points are kept in mind:

- The Shāfiʿī Imām should perform three Rakʿāts of Witr Ṣalāh with one Salām. One of the great Shāfiʿī scholars, Imām Nawawī (R.A.) has stated in Sharḥul Muhaẓẓab Pg. 3, vol. 4, that this would be more meritorious and more preferred. If he performs it with two Salāms then the Ḥanafī follower’s Ṣalāh would not be valid.

2. During the first 15 days of Ramaḍān, the Ḥanafī Imām should perform the Witr and from the 16th night, the Shāfiʿī Imām should perform the Witr Ṣalāh (Sharḥul Muhaẓẓab Pg 15 vol. 4)

3. As far as the Ḥanafī followers are concerned, they should recite the Qunūṭ of Witr after Rukuʿ when following the Shāfiʿī Imām and they should recite the Qunūṭ without lifting the hands.

If these few points are kept in mind, then it would be very easy to combine these two Jamaats and both sides will gain the full reward of Witr.

Fatāwā Darul Uloom Zakariyya vol. 2, Pg. 422-426

And Almighty Allah knows best.