We can do a slight change to your workflow…
After the first read range, you will be getting the data onto a datatable.
So to check whether that datatable has any data, you can use the below if condition.
DT.rows.Count > 0
If it is 0, that means you dont have data and you need to switch to the other excel. So in that section of the if condition, use the other read range activity to read the data from the other excel file.
Also, I think you can simplify this more if you use the workbook activities of the excel so that you can omit the excel application scope.
If you want to use it, the I suggest in the if condition, use a boolean variable and based on that, do the next read range below the excel application scope which is already there to make the flow simpler and easier to understand