InfoPath Dev

I have 2 separate repeating tables in a repeating section. Each table contains subtotals in their footers(which work fine). How do I create a grand total that sums the 2 subtotals?
I am currently trying the following expression (in which each of these items is an expression)
my:StandardLaborEst/my:CurLaborCostTotal + my:StandardODCEst/my:CurExpCostSubtotal

What happened? Is your solution not working or are you looking for a better solution?

Please give more details about your XML.

No errors shown, just getting no results in the Grand total calculation. The subtotals show results but the grand total does not update and just shows $0

Hi,
1. Select Expression box in the Controls Task pane.
2. Click on “fx” button in the Insert Expression box.
3. Enter Sum(Field1)+Sum(Field2)
4. Click on ok.

Field1 is the column in First repeating table.
Field2 is the column in Second repeating table.