Is a connection between the double garage on the roof and the neighbor's garage necessary?

Hello everyone,
our new double garage was delivered recently. We are now wondering if it needs to be sealed at the roof with the neighboring garage. Our garage is about 10cm higher than the neighboring garage. The team that installed the garage said that sealing by a roofer is not necessary. I am wondering what happens to the rainwater that flows on the roof into the gap between the garages?
Best regards,
Franziska

Unfortunately, I do not have a concrete procedural suggestion for you, but I confirm that, in my opinion, this matter should not be left to the free will of the water, and I thank you for the unusually good illustration of your question, especially for newbies.

Hello Franziska, I would lay a track there that connects both. Certainly, it is not necessary. But with it, it is a clean matter. You will have no runny noses due to moisture, the neighbor is also protected from wetness. After all, now less wind comes there, so it dries worse for him. Since you [angebaut habt] and are also taller, you are somehow obligated now.