I am not very familiar with the CalculiX configuration, but I find this in the adapter documentation:
The adapter supports quasi 2D simulations when the z-direction is ignored. If you set the preCICE interface dimension to 2, the adapter will map data from the CalculiX 3D simulation to 2D space and vice-versa. The 3D simulation should be made of solid elements (or shells) of unit thickness.
Maybe @IshaanDesai (or actually, @boris-martin) knows a bit more about this. I find some related topics here as well, such as Perform 2D simulation with Calculix.
As far as I know, we have no tutorials for this. Contributions welcome!