I've been out for a while, but I'm back. I'm running 1488 now.
Without "Album Art Tagger" script, tagging cover images is almost instantaneous. I'll contact the script author about this.
I have added cover images to 3 albums manually and on 3 occasions did selecting one image result in MM registering 2 images.
Here are "Add Artwork" options I selected:
Pressing Ok results in 2 cover images being recorded:
Here's the debug log:http://notepad.cc/niotosma95