Actually divide the property but both have the same building rights

It brings me income.

I do. Whether it benefits the OP depends on his desire for independence. I prefer maximum independence, which is naturally already limited with a semi-detached house. Depending on the development plan regulations, there are many levels of dependency. One could make a shared driveway, shared parking spaces, or the mentioned division according to the WEG (Condominium Act). Many details are missing for better assessment:
- How old is the development plan (site coverage ratio, floor area ratio, and ancillary structures)
- What else does it regulate (ancillary structures, heights)
- Exact terrain elevations
- What is important to the OP (priority list)
Here is a suggestion with the preferences revealed so far (equality and maximum south garden):
[ATTACH alt="Lageplan.png"]51589[/ATTACH]

Since site coverage ratio and floor area ratio are ratio numbers, the possible structural utilization of the plots naturally depends on the plot size. Fairness is therefore only achievable if not only the building envelope is divided centrally, but also the subplots are equal in area.

My wish is to have as little in common as possible except the partition wall.

Also attached is a proposal from the owner.

Do you see any problems with it?

The owner has just informed us that he will only allow a division after the notary appointment and that he bears no responsibility for the division or the lack thereof.

Thank you very much for the comments and suggestions.

but you cannot predict the future.

My wish is to have as little as possible in common except for the partition wall.

Well then, since adjacent driveways are not allowed:
[ATTACH alt="Lageplan2.png"]51613[/ATTACH]

I find that stupid.

Yes, if you can't rely on the construction partner. It depends entirely on the contract design.