I can think of two ways to do this, and they are practically the same.
1. Create a three column secondary data source that is populate from the SharePoint list. The first and second columns will be code and description. The third column will have a formula for the default value, which will be:
concat( code, ": " , description);
2. Do the same thing, in SharePoint, then you don't have to add a secondary data source to your form.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge