You are much better off trying to join the tables at the SQL level for what you are trying to do. The parameter cannot switch what is considered the 2nd source to be the first (which is why you are getting the Attr)
One option if this isn't possible is two worksheets on a dashboard for each data set. When the parameter is set to the one datasource field, the other worksheet collapses due to no data. Search for Joe Mako's post on this.