var result = parseInt(Subform1.Question1.rawValue) + parseInt(Subform2.Question2.rawValue);
Since the values returned from the radio button groups are strings, you need to convert the values to an integer using parseInt() before you can add them.
However: be careful where you are doing the calculation if you are not defaulting a value for the button groups. If none of the buttons are selected in a group, it will return an empty string for the value, which parseInt will convert to "NaN" (meaning Not a Number) and you will end up with a meaningless result. Either: have a default for all button groups, or check that a valid number exists for each group before doing the calculation, or put the calculation in an event that will occur after all selections have been made.
Sorry, read your initial post again more closely. If you're going to use a numeric field to display the sum, then just put the following in the calculate event of the numeric field:
The numeric field will throw away any value that isn't a number, so it won't display anything until you actually have two valid selections.