So where to start at this big project? The first experiments were performed with different steppers to get some experiences, which are suitable for this clock.
We started with the Sonceboz 6407 stepper, which has two concentric shafts and looks very usable for this project. However the price of the motor, even when buying 125 pieces (around 27€ each plus taxes) was way to much.
Another test was performed with the VID29-05 (or the clone BKA30D-R5), which you can get for less than $4 at Alibaba. This motor uses many gears to connect the two individual steppers to one shaft. This leads to an enormous increase of annoying sound while running. Even one stepper is to loud for my personal feeling. So, no chance for the VID29-05.
The next attempt will be with two X27 steppers, connected by two external gears (probably laser cut) to one shaft. The motor costs around $1,50 and the construction is much similar to the Sonceboz 6407.