I believe the "Newer" way to handle this is to add a cell that has your Sum formula on each workbook.
Then add the "Watch Window" to Excel.
This allows one to monitor multiple cells in one location.
This has less chance of errors and confusion over what and where things are selected.
I've been using this more and more.
One can use the Status bar in many variations. But, it's probably not the best way.
Sub Demo()
Dim x, y
x = WorksheetFunction.Sum(Range("[Book1]Sheet1!$A$1:$A$10"))
y = WorksheetFunction.Sum(Range("[Book2]Sheet1!$A$1:$A$10"))
Application.StatusBar = "Book1: " & x & " " & "Book2 " & y
End Sub