Create a new column that concatenates the two values and use that column as your DisplayName value of the dropdown. I'm not sure you can consume calculated columns into InfoPath through list data connections, so you may need to create a simple SPD workflow that sets the value of this new field (make sure it's hidden from the user by hiding it in the content type) to the syntax you prefer. You would use the Build Dynamic String action in SPD, and the dynamic string would like this:
[%CurrentItem:ID] - [%CurrentItem:Description]
Just choose the two fields by clicking "add lookup to body" within the dynamic string builder.