What you want to do is not possible as each map has its own Map ID or Family ID. Garmin BaseCamp/Mapsource does not allow to have the same Family ID more than once - see the warning: Segment codes from LaosandNT.img exist in other files too. So you can only show the North Thailand map and the Laos map either as separate maps or as a combined map.
Not sure why you want to combine the maps but, in case you want to make auto-routing from one country to the other possible, this will not work as the connecting points between the two countries do not have the special code embedded in the maps which allows to do this.