I'm using Excel 2019 & Office Tabs 13.
In single window mode, I cannot have two instances of excel unless I open via right clicking the OfficeTab worksheet name and selecting "New in New Window" or "Open in New Window". This is unsatisfactory as I have written user defined functions in an add-in and on the second instance, the functions cannot be "found" and the full path to my add in is shown in the formula bar. If I press [F2] to edit the formula then [Enter] "#NAME?" is shown in the cell.
If I am NOT in single window mode, I can readily have two instances of excel, each on a separate monitor BUT the tabs are duplicated in each window regardless of which instance the file is opened in. This is extremely distracting & confusing so as to be unworkable.
Previously I was using Excel 2010 & Office Tabs 13 with no such issues.