These are views in Excel. It's actually the same workbook but you are just "viewing" it in two separate windows. It's handy sometimes if you have two monitors and would like to view two separate sheets of the same workbook side by side. To get rid of this view, just hit the "x" on one of the windows for the views. Once you get down to just 1 window for that workbook, it will be normal again without the colon and the number. To manually create these if you are ever curious in the future, you can go to View on the ribbon and then click New Window. Hope this helps!
