@René ID,
I've built a sample based on what you've described. The first station allows 12 items to go through untouched to the second station, where they are processed. Then, the next 12 items are processed at the first station and are untouched at the second station. I used a "Count" label on the first station that tracks how many items have passed through the station, which is used to indicate whether an item should be processed or sent along.

The "Complete" label on the items is used to indicate whether an item has been "processed" or not yet. The second station is the station primarily concerned with this label as this is how it knows whether to stop and process an item.
All this is done using labels and triggers. You could do the same thing with Process Flow. Hopefully this gives you some ideas.
rene stations help.fsm
If you attach the model that you're building, it will make it easier for us to understand the process and provide an accurate solution.