ᐅ How to seal a garage that is attached to the neighbor’s garage?

Hello everyone.

I am currently in the middle of construction and have a question regarding the waterproofing/drainage of the garage. Both garages are built adjacent to the property boundary. Since the neighbor’s garage is already completed and mine has just been built, I am wondering how to finish the 60mm (2.4 inches) gap between the two garages at the top to ensure rainwater is directed away from both garages, preventing moisture from entering the gap.

In the attached photo, you can see the neighbor’s garage on the left. Once my garage is finished, it will be a few centimeters (inches) higher.

I was considering using a metal flashing sloped towards both garages so that rainwater can be drained away from both sides and the gap remains sealed.