You confuse me a bit . Please correct me if I wrongly understood your problem .
You created a report in Desktop via excel which is stored in SharePoint . Then you published the report to Service . Then you wanted through Get data to connect to the dataset which original data source is the excel file in Desktop , it worked ! You wanted to add one more , but failed .
Similarly , if other dataset is added first , it is successful . Then add one more dataset which data source is excel , it will fail , right ?