Are you are doing data entry in the forms and if so, are these totals for the data after the changes have been posted back?
If so the easiest way is to query them out
Look up the aggregate function Sum in SQL
If not and you want the current total if the changes were to be posted then you are going to have to cycle thru the dataset, add up the money, and then store it in class or struct that all three forms can see, youll also need some way of totalling up (one or both forms), if you haven't opened the datasets.
Depends on your UI, but it would be a lot easier if you only had two forms, one to do both detail sets and a total for each, and the Totals function could be a panel on the bottom, or a pop up or some such.
You could also instead of havng two (or even three datasets) just have one, with all the different doings in it.
If you want code, you might want to mention which language you are using in VS2008...
I suspect you are about to find out why I loathe datasets and prefer not to use them...