I've played around with this a bit and found that with my set-up: (which would be fairly typical. I believe, because I just reinstalled the OS back to the factory settings)
normally it would be impossible to have more than one personal.xlsb open at the same time UNLESS you've defined it as a Shared workbook - have you done that?
If you open different workbooks in the same instance of excel it will only open the single instance of Personal.xlsb. But if you open several workbooks in different instances of excel then it will try to open the Personal.xlsb once for each instance.
When you have separate tasks in the scheduler to open different workbooks it will open each workbook in a separate instance of excel hence the multiple instances of personal.xlsb.
One way around this is to have a single task that opens a blank worksheet that has this macro in the ThisWorkbook module;
Private Sub Workbook_Open()
This will open all the workbooks in the same instance of XL and hence only one copy of Personal.xlsb
Is this a viable solution for you?