Join the two .img files to one .img file using using as "Mapset FID" the mapID of the map with the most complicated TYP file (map with no roads). Use PID=1.
Then Split the new .img (mapset) for Mapsource, using again as "Mapset FID" the same map ID as you did above.
Mapsource will now read this mapset as one file and is going to use only the TYP file of the map, whose mapID you gave to the mapset.
Open this TYP file and add (copy-paste in the center window) all the elements of the other map's TYP file. Now Mapsource will draw both maps using one TYP.