Not sure if you still need assistance on this. However, typically it's not possible to chart values from two different worksheets. Depending on what you're trying to do exactly, filters could be put in place on each sheet individually and not on the third. If you're able to post a packaged workbook (twbx file) I might be able to help you further.
Are you wanting to chart values from two Excel worksheet data sources in the sane Excel workbook, or from two Tabeau worksheets within the same Tableau Workbook? Depending on what you want to do, the approach is a bit different.
Looking forward to hearing from you here.