The two osm files probably overlap a bit and, if so, so do the two images. It is also possible that one of the two images (or both) are actually too big. In either case I don’t think that it will work very well.
Imho you have three options:
- Merge the two osm files together with Osmosis and convert the resulting single osm file into a Garmin image and gmapsupp. You’d probably also need to use Splitter to convert the osm file into sections that can be converted to Garmin images as a single image is probably too big.
OR
- Go to http://garmin.na1400.info/routable.php, enable manual tile selection and select the tiles you’re interested in.
2.1) Enter your email address and hit the ‘Build my map’ button. Then you wait until you receive the download link for the complete map.
OR
2.2) Download the tiles listed on the left-hand side and merge them together with Mkgmap as you did in your post above.