How to adjust the assembly details of two adjacent stringboards using the link parameters section in the properties menu.
As usual with parameters, when no selection is applied you will be editing the parameters for all string to string assemblies in your stair.
Alternatively you can select any specific stringboard and modify the parameters just for that in isolation.
When adjusting the whole stair and all stringboard relationships, the priority can be adjusted between priority to the lower stringboard, priority to the upper string, or to a mitred joint.
When adjusting a single stringboard, the priority naming changes, your options now are overpassing, underpassing or again a mitre. The effects remain the same however.