In a Celonis KPI (like a barchart) you can only select one dimension within a Component KPI. For a stacked barchart you need to design a column chart with a dimension (e.g. time) and create 2 (or more) KPIs which you want to stack (these are your two stacked dimensions/parts if you want to say so). Afterwards, in the component options, you need to choose the data series of your KPIs from the dropdown and enable the Box “stacked” in the data series settings.
Example: Create a barchart and choose the Eventtime as a dimension and set up two KPIs case count and activity count. These KPIs are the different parts you want to show stacked in the bars, so you only need to change the component settings as described above.
Hope this is now more understandable!