You have two options:
1. Create the main report off of one table, then use a subreport to bring in the other table's data for each person. The drawback of this method is that you won't be able to sort the rows from both tables together.
2. Write a Command (SQL Select statement). This command will be a UNION statements that pulls the data from Activity in one query and unions it together with the data from History to provide one data set. The fields must be in the same order and data type in both queries.
If you don't have good SQL skills, you can write the report for one table, go to Database>>Show SQL Query and copy the query here along with information about which fields from the History table map to the fields in the Activity table and I'll help you write the query.
Also, if you haven't ever used commands, you can get more information about the right way to use them here: Best Practices When Using Commands with Crystal Reports
-Dell