Crystal Reports Pulling Data from Sub-report

Message

   Hi,
My CR knowledge is very limited so please bare with me. 

I have a report that have a running total of formula fields (ie. SubTotal_A) and sub report that have a running total of a different formula fields (ie. SubTotal_B) . I'm trying to pull back this SubTotal_B from the sub report and add it to SubTotal_A to make Total_A+B. Both report uses the same store procedure for the data. They are in seperate reports because they sum the value in different ways. 

-----------------------------
main report
GroupA
   DataA1  ValueA1
   DataA2  ValueA2
-----------------------------
                SubTotal_A
-----------------------------
subreport
    DataB   ValueB
-----------------------------
                SubTotal_B
-----------------------------
                TotalA+B
-----------------------------

My question is:
1. How can I pull the SubTotal_B from the sub report back to the main report so I can do some manipulation to make TotalA+B?

I have tried to add the same formula fields that's in the sub report to the main report, but it prints out as 0.0 in the main report and not the same value as the sub report. Is there another way of doing this?

Thanks,