Potential / Experience - building two houses at the same time?

everyone only told us that there are no major savings potential.

If middle houses want to build with a basement and end houses both without, but with different roofs, then that is likely entirely true. However, the greatest savings potential is precisely here: building with a basement and without a basement next to each other simultaneously significantly reduces unnecessary extra effort.

built simultaneously

That would be way too easy...

If you do any topics together, you should definitely set clear rules in advance. We shared the construction power box, power supply contract, toilet, water, and did the earthworks at the same time, and there was only trouble with that. We also did not fence the plots individually, which was a bad idea in hindsight because the craftsmen were not always clear or did not care where things belonged. And if you then have the excavation from the neighbor on your property, that can be quite annoying and cause friction points, even if it was not intentional. We would not do such a constellation again.

Yeah, difficult, will be insignificant. A few thousand if it goes well.

are they only neighboring plots or a semi-detached house; and are the houses clones / mirror clones or different?

No semi-detached house and houses probably also different.

Doesn't bring much, just coordination hassle and dependency. Site setup with crane and all the bells and whistles was €3000, I think. That would be the maximum savings in my opinion. You can't expect bulk discounts because of two extra toilets.