Plot selection for a semi-detached house - Which one would you choose?

Should it not be possible to have two semi-detached houses built by different builders, for example one made of solid material and one with a timber frame?

I am not fundamentally against that ;-) However, I am of the opinion that if you want to understand the "shared" wall as a seam (and not as a friction surface), a common planner is the best guarantee that the whole thing will fit together. Even with different construction methods, in my opinion, it is good "if the same brain sits between both ears". With one site manager it is even possible to "partially basement" the semi-detached house. And the optimum, of course, is when conjoined twins are also identical ;-)

... unlaid eggs...

Of course, the restrictions of the valid development plan apply. However, these are often not particularly restrictive in terms of appearance. Sure, heights are basically always specified, as well as the floor area ratio, which already sets certain maximum dimensions.

In the accompanying documents, I could only identify the general specifications you mentioned (floor area ratio, maximum height), as well as the obligation for green roofing. I have now explicitly inquired whether there are further requirements for visual alignment.

From your excerpt of the plan, there is no indication that this "ED" could not also be developed, but in this specific case, the access situation makes a division difficult.

Upon inquiry, I was confirmed that the said plot could be developed with a duplex. They were only surprised as to why the only single-family house in the building area should serve as a duplex :)

In den begleitenden Dokumenten konnte ich bisher lediglich die von dir angesprochenen allgemeinen Vorgaben (Grundflächenzahl, Maximalhöhe) feststellen,

I would have liked to offer my practiced eye on the development plan, but it seems to be available for inspection only on site - not online.