Hi,
If I understand your requirement correctly :
a) f1 and f2 are 2 fields then :
You need sum of f1 and f2 If this is the requirement.
write a formula column that returns sUM(f1,f2). But this shall be running total i.e if f1 and f2 change value then formula column value also should change. Hene all the 3 i.e f1,f2 and CF_sum should be present in the same group. (else you may get in Layout Referring to below 'Frequency)
b) Add the totals of f1 and f2:
As you already have summary columns on f1 and f2 . Write a formula column CF_1 to return (SUM (S1,S2)) where S1= Sum(f1) and S2=SUM(f2).
This can be at report level also.
Hope this helps.