Hi Lucian and welcome,
In the InfoPath Designer, go to the taskpane and select the secondary data source that you are retrieving information from. Right click on the field that holds the value of the data downloaded from your SQL database. Choose Properties. Select Rules tab. Click Add. Click Add Action. Select "Set a Field's Value". Click the icon next to the Field and select hte field to set from the main data source. Click on the fX button and insert a field. Choose the default (same as the secondary data source field). Click OK until you escape from dialog box hell. Preview the form.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra