Official comment
You can upload multiple separate missions together and as long as there is enough overlap between the missions it will all get stitched together as one.
If for some reason they can't all be uploaded together (like them being too large), you can create a Location Map within Maps Made Easy to allow both of the map overlays to be shown at the same time.
Merging the GeoTiffs can be done using QGIS or GDAL tools.