Two-family house with 2 owners

And what if one of the parties doesn't have the money for it at that time? Or if you can't agree on replacing the heat pump (one wants to replace it before it breaks down, the other wants to wait until the bitter end)? There is a lot of potential for conflict. I wouldn't want that. Owners can also change at some point and then your friends won't be living next door anymore. What will you do then? I wouldn't want that.

These are of course all points to consider.

The house contains a shared technical room, which must be deducted from the respective living area in equal parts and is accessible from both sides through a door.

The access would be basically a central door to a central room. Your two front doors would be to the left and right of it.
You want this because you think you’ll save on technology once, since you don’t each need your own technology, but a shared one. The technology in question is the expensive heat pump, which then has to be larger than two smaller ones for each of you.
Everything else must be provided separately by each of you: electricity, meters… honestly? How much do you save? In my view, there are conflicts with maintenance: on one side, for example, 4 people live there, the other side only 2, and uses only half as much heating as the other side. Then there is an imbalance, which can negatively affect the friendship.

I wouldn’t do something like that with friends: neighbors can become good acquaintances, but friends don’t become good neighbors.

At that time, I had built a 2-3 family house with/for my parents, but unfortunately did not separate it properly. When things changed many years later, I had it divided, which was possible with a self-made, simple sketch. As described here, each unit must always function completely independently, i.e., everyone must be able to directly enter the designated shared rooms. Here, for example, a utility room that must be accessible from a common corridor, etc. Each of the negative points mentioned here has its justification just as the positives do. I would actually calculate (or have calculated) the financial advantage for each side as concretely as possible and do this completely open-ended to then see whether it is otherwise not possible or whether it is (not) worth it to me. I then sold my parents’ attic apartment at that time for very little money (which I would never do again) to solve other problems, and that’s when the trouble started; I sold the basement separately (without trouble). Ultimately, we even had to hire an external property management company because the lady in the upper floor simply stopped paying anything and really blocked everything like in some sort of a war of roses. In the end, I also sold my main apartment (at a significantly better price by then) and created something new elsewhere. I would definitely get comfortable with the idea and clearly address beforehand that anyone can pull out at any time and sell/rent their part or whatever, so also clearly name the worst-case scenario in advance. Basically, it is nothing other than a small multi-family house, which can bring advantages and disadvantages. How it will be for you cannot be foreseen, not even for yourselves, because people and things simply change (both positively and negatively). Ideally, truly clear rules are agreed upon and written down beforehand that clarify the area of coexistence as best as possible. “Let’s wait and see” or “We’ll see then” are approaches I would want to avoid. There are always technical possibilities for clean accounting, and implementing that is not a big problem.

I would definitely come to terms with the idea and also clearly address it beforehand that everyone can opt out at any time and sell/rent their share or whatever, so the worst-case scenario is clearly named in advance.

Yes, good idea. Nevertheless, people might not like the worst-case scenario ("hey dear ones, we are renting out because we want to move and change") and it might cause discord.

This plot belongs to a new development area, some measures have already been taken by the city before the sale. The soil report was created, a cistern was installed, the sewer connection was laid one meter onto the property, and a geogrid-reinforced bedding cushion was made.

Soil report for this exact plot or only for the area as a reference? Those are two different things. The plot would also have to be of a corresponding size or have a large floor-area ratio/building floor-area ratio so that two families can build their residential units on it. Can't you just ask the municipality if a semi-detached house would also be possible? Open development applies even to a semi-detached house and it should be legally feasible.

Also interesting would be the worst case, when on one side the hot water tank is emptied by a lot of bathing..

What would surprise me a bit is if the building application with separate HA rooms were approved. In doing so, you would ultimately be creating a semi-detached house again, which is prohibited according to the development plan.