Demodex Mange will normally start around the eyes, nose, and lips of a dog. Sarcoptic mange will be all over but the dog will itch like crazy. Allergies start on other places of the body like ears, chest, and paws. This does not look like any of those so that's good. If it is just on tail it could be the way your dog lays down. Does she sleep in a crate while your gone or at night? I have issues with a few of my dogs and it's just how they lay the hair thins out on the tail. Also if your dog beats it's tail against walls, crates, or furniture they can lose hair. Either way it doesn't look like a serious problem. I would order some nustock
Nu-Stock - Topical Skin Treatment for Animals.
And apply it a few times a week to help with growing back the hair. I know this stuff says it's for mange too but it helps grow hair back. Nustock stinks so I would put it on when you crate your dog for the night or leave for work. Or you can do nothing and just let nature takes it's course. I didn't treat any of my dogs because it's not a big deal and doesn't need to be treated you can just try to grow the hair back.
Dry skin could also be a factor, you can give your dog a 1000mg fish oil pill once a day in the food to help with that.
BTW most dogs carry the demodex mite like you and I have mites on our eyelashes. (yes you have mites on your eyelashes!! lol) But in a healthy dog the immune system keeps them in check. In a dog who has a weakened immune system they can take over and that is how they get an outbreak. So worrying about it getting on your other dog is not a factor since it's an immune system problem not the mite itself.