Thank you for the reply,
the second data source is coming from an XML file. I can combine the values of 2 data fields with the concat command in a calculated field, but then I can not use it in the pulldown menu. Right now I worked arround the issue by letting my users first lookup the data for the correct information and then to let them use the looked up information from the pulldown menu. So in a way they can find the correct information in 2 steps instead of 1.
But thank you for the explanation that the dropdown box will not allow to be populated with aggregated values.
Marcel