yeah, I wouldn't worry about them sharing the shed. that's normal.
hmmm. at some point there was an injury. it may have been awhile ago, and possibly under old skin. geckos are fast and it's not easy to inspect them fully all the time. it's possible she rubbed it into an injury during shedding.
black marks also indicate burns or fungal infection. based on the location I would say a burn is unlikely.
I don't *think* it looks like a fungal infection, but it's hard to be certain. I'm not a veterinarian.
I'd keep an eye on it til goes away. if the area swells, oozes, turns any other colors, or just looks "worse" she needs to see a vet asap. as it is now, if it were my gecko I'd just keep an eye on it for awhile. it might not get better until her next shed cycle, when you'll probably see the color get duller if it is a scar.