When a macro for Workbook1 starts, is it possible for a different workbook to start a macro before the macro for Workbook1 finishes?

Specifically:
I have a workbook (Common.xls) that is referenced from several workbooks.
Some code in Common.xls:
Public gWBCalling as Workbook
Sub MyMacro(wbCalling as workbook)
    set gWBCalling = wbCalling
    Call Another_Sub
End Sub

Sub Another_Sub()
    .... several lines of code that takes 5 seconds to execute.
    With gWBCalling
        .....
    End With
End Sub
Is there any way at all that this could happen?
1. Workbook1.xls calls MyMacro(ThisWorkbook)
2. MyMacro sets gWBCalling to WorkBook1
3. Calls Another_Sub
4. Workbook2.xls calls MyMacro(ThisWorkbook)
5. MyMacro sets gWBCalling to Workbook2
6. Another_Sub gets to "With gWBCalling" (which is now set to WorkBook2)
Which would trigger an error on the next line (or at least use the wrong workbook).