You can specify it in the Output Tool which sheet to write too. Keep in mind though that it may throw an error after writing the first sheet. You will want to add a Block Until Done tool or Control Container for each output so the workflow finishes its write before moving to the next path or container. If this isnt done, you'll get an error saying the file is already in use. Below is an example of a workflow that outputs to one Excel workbook, but writes to different tabs within the workbook.