question

If you need 2 parallel station, then you build in 3D-model in the conveyor system another parallel conveyor next your station. This conveyor branch gets items too.

The parallel stations stop the item OnArrival if the other parallel station doesn't contain an item. The next OnArrival action is to resume the other station. This Trigger actions are coupling the two stations, thus they are starting at the same time. You can place the two station in a line and you let pass the first station the first item if the second station is empty. The second station in line stops the item OnArrival trigger. The second item arrives at the first station. This arrival let the item on the second station resume and be processed together with second item.

If the second item shouldn't be tested twice at the second station, the arrival condition and process time condition checks the statistic input of the station such that only every second won't be stopped or processed. This is done by the modulo operator "%". The modulo operator does an integer division. The result is the rest of the divison. If you divide the input by 2, you get by this operator 1 or 0 as a result. This can you use to decide wether the item must be tested or not.

You can assign a label at the items, that is checked, if the item must still be tested. This might be easier to implement than the mudolo operator approach.