In your 255W, only gmapsup1.img will work in the SD as the second image, i.e. x:/Garmin/gmapsup1.img. If it doesn't work then update the fw. You can only have two image names in your SD card with that unit, gmapsupp & gmapsup1 not gmapsup2. If you need more maps, you can use MapSource versions and combine them with MS up to the 4GiB FAT32 file size-limit provided that you also don't exceed the total number of both map tile and individual map limits. That way you can (in theory and subject to the other 2 limitations) have up to 8GiB of map data as 2 X 4GiB image files in the SD.
Using unlock codes is going to run you into trouble with FID/MID of 2200 or above. Read [Only registered and activated users can see links. ] and just use [Only registered and activated users can see links. ] to unlock such maps.