From what you describe I assume that when you say "dropdown box" you are talking about a Value List. If so, the simplest solution is probably to make the concatenated string as a field, and then use that field in your list.
what keywords is saying is here is the structure of the list source table
calculation_field = text_field_1 & text_field_2
List first value is pk_field
List second value is calculation_field
show only calculation_field
Thanks for clarifying. That's exactly what I was trying to say, but much clearer!
Thanks a lot for your answers. I come from SQL Server and MS Access development and as I see until now, there are some things working different (but as I think about it, better) ;-)