Bauhaus style or cube examples

Duplex flat roof in the Google image search does lead to some results.

Of course, we have already searched the wide web, but were not really happy with the results

Then I fear that this is actually not really your desired style..

... then maybe you should specify more closely what made you unhappy ... because no one here has a crystal ball.

I’ll give it a try. Most flat-roofed semi-detached houses on the internet have a staggered floor (2.5 stories), which of course looks quite good. But we will only be able to afford two stories, and most flat-roofed semi-detached houses look like two cubes glued together side by side. We were simply hoping for some architectural elements to loosen up this cube a bit (protrusion, recess, color accents). Anyone can google, that wasn’t the intention – we were hoping for suggestions from people who had similar challenges (possibly their own example photos), that’s what a forum is all about. But I notice that there aren’t many people here who have built with flat roofs.

If you don't want two cubes glued side by side, then avoid the visual separation on the front. This will elongate the appearance. You could set accents with a combination of carport and roofing at the respective house entrance and/or balconies / terraces. A bit more clever would be to dispense with symmetry around the central vertical axis of the building front. Is it already clear who will move in where? Are the needs of the residents identical?

We simply hoped for a few architectural elements to loosen up this cube a bit (projection, recession, color accents).

Such attempts at a "poor man's variety" usually backfire or at best yield results that would be described in job references as "made an effort." If you try a "Bauhaus" without having a "green thumb," you end up with Hornbach style. Already trying to predestine in the building composition that it "must" look good often leads to too little and never to satisfaction.

But I notice that there aren’t many people here who have built with a flat roof

Because it was often desired and then abandoned when the owners heard the prices. All the examples you listed of measures to prevent a flat roof from looking like a roof that was left out cost money; in addition, the flat roof (alongside the hip roof) is a roof shape not suitable for the thrifty.

We are talking about a cubic flat-roof house (is required in the development plan).

What exactly does it say there? The horror head cinema of laypeople when reading "flat roof" in a development plan, that without forced acrobatics (or antics) it would turn into a deadly boring shoebox, is at least rarely reached in reality.