ᐅ Carport built on the boundary line on both sides... how does the 9-meter rule apply in this case?

Be careful with the height. I think you will already face difficulties here... I believe you are not allowed to build more than 3m (10 feet) higher. And yes, it counts as a boundary construction.

Thank you for the assessment. We will only reach 3m (10 feet) about 2.5m (8 feet) away from the property boundary, so that should not be a problem. We are more concerned about the length of the boundary construction...

In our area, a double carport can be built on the property line and is even exempt from a building permit / planning permission if it is under 40m² (430 sq ft). That means approximately 6 x 6.5m (20 x 21 feet).