Hi Leslie,
If these are two tables present in your universe, than easiest way will be two join the two tables in universe and create objects from those tables.
You can use these created objects in the webi report.
If these are coming from two different queries in the report than you can make use of merge dimension concept..
Details of using merge dimension in webi are in below link:
http://scn.sap.com/docs/DOC-49941
Thanks
Gaurav