Sounds like you want to club two properties into one and then display it .
May be you need to use a report definition and then use join to fetch data from other table and post that club the properties to store in a place holder via an activity and use that as source of dropdown.
For autocomplete, you can do that almost in similar way. Use a RD and get data from two sources. Then use both the properties as source in auto complete.
Check out the example given in UI Gallery :Screenshots below :
We could achieve this requirement using join features in RD. We can choose either Data Page or RD to pull the data from Databasetables which are associated to datatypes (classes).
1) If we choose RD: Please refer the classes which are required to be join to pull the data. (Col1 from Datatype1 and col2 from DataType2). here we should provide filter criteria to fetch the data from second databasetable. In pre-load activity of the screen, please try to set both the col1 and col2 values to one placeholder param or property. Provide that as a source property for the dropdown field. OnChange also we may need to run the activity which is called in pre-loading of the screen.
2) If we choose DataPage: Choose load activity option. After that the process would be same as above. Provide DataPage as Source for the dropdown. Group By feature is probably not required for your req. If required, you can group the data by selecting Group Items checkbox if are going with datapage option.
Unlike load activity, we may achieve it using lookup of two classes and get the data into datapage. Post Loading data mapping features are available in DataPage.