This a minor Microsoft Office Excel 2010 and Excel 2013 bug.
The second XLS file is waiting for the first XLSX file to exit out of cell editing. Once you hit enter and leave cell editing mode, the second XLSX file opens.
There is no fix as far as I know since it existed in Office 2010 Excel and continues to exist in Office 2013 Excel. There's another discussion about not being able to open a second Excel spreadsheet here as well. Same conclusion.
You just have to press Enter (to leave cell editing mode) before you open the second Excel spreadsheet.