Without further clarification, I am having a hard time understanding what your exact application will be and so I am not sure if I am providing you with the information you need.
The Switch expression would still work regardless of whether or not you use different fields.
@WhichDataSetParameter = "Use Dataset 1",
@WhichDataSetParameter = "Use Dataset 2",
Or if you meant that the parameters would be different based on whichever dataset was used, then you can can extend the boolean expression to determine the dataset:
Not IsNothing(@DataSet1Parameter) OR Not IsNothing(@DataSet2Parameter),
IsNothing(@DataSet1Parameter) AND IsNothing(@DataSet2Parameter),
NB. The above would really be best as an IIF statement, but I kept the Switch simply as that's what we'd been using so far. No matter how you go about the solution, conditional expressions will allow you to select between different datasets which is dependent on the logic you provide.
I would be happy to provide more details if you are able to provide a more inclusive breakdown of how the report logic works to determine which dataset you would elect to use.