It sounds as if she is resource guarding you. Are you certain they get along when no one is home, or do you just assume so because no one is hurt? They might still get in scraps while you're gone if you think toys and such could be an issue. 

Best thing to do is to take away anything you think might cause conflict. Food dishes, toys, etc. just to be safe. Also, the more opportunity she has to guard things, the more she'll do it and it will be harder to break.

As far as resource guarding YOU, Comet's mom is right. Ignore the bad, reward the good. If you're petting her and she growls/shows teeth, stop immediately and redirect your attention. When she's behaving praise her and give treats. Food helps mitigate these situations quite well – if she learns anytime the other dog(s) are around the both of you, yummy and fun things happen, she'll stop seeing them as a threat.

It's also about management.  Right now do your best to prevent negative interactions when you can't spend time working on it with them. If you know you only have two minutes to spare to let her out and what not – do what you can to set things up so she doesn't have the opportunity to have a negative interaction with you or the other dog. It's kind of like potty training in that sense – the less chances they have to pee on the floor, the easier and more successful housetraining will be. 

 DO NOT punish any aggressive displays of behavior. These are her warning signals that if she is pushed too far, she will snap. When you punish the behavior (startling, yelling, whatever) to her, you're punishing her warning. Then you can end up with a dog who gives no warnings before she snaps. They aren't as complex in their thought patterns (I don't know if that's the right word!) We want to discourage the behavior and the reason they're doing it. To them we're just discouraging the behavior but not effectively dealing with WHY they have that behavior. It's like anything – dog's who are destructive are often bored – we can punish them for chewing and it might work, but if we don't do something to solve the boredom, it's likely they'll redirect their energies onto something else we don't like. She might go from growling to nipping instead. 

I haven't read this book but I have seen it recommended many times – it's called "Mine! A Practical Guide to Resource Guarding in Dogs" and is by Jean Donaldson. It's about managing resource guarding with dogs. She also has a book called "Fight!" which is about dog on dog aggression.

Ian Dunbar also has great books and methods of training you may want to look into – he's a positive reinforcement trainer. 

You might also find it helpful to do some searches on dog aggression on breed specific forums. I'm on a Pitbull forum and because PitBulls have such a bad rap as it is, the emphasis on that forum to effectively deal with any sort of aggression that happens with ANY dog is really big. There are TONS of topics on these types of forums that can help you distinguish what type you're dealing with and how to manage it.  

Dante has A LOT of issues. Partly because he's had no fight or flight since he was 8 weeks old, and because the loss of his leg was a result of being attacked. He's leash aggressive and resource guards, he's also very fearful. Only towards other dogs and typically only new dogs (but he does occasionally get nasty at home with his sister) and we're working really hard to manage it and improve it, and he's doing well. He'll never be a happy go lucky love every dog no matter what, type of dog, but we can get him to where he tolerates a lot more than he does and isn't as afraid as he is. 

Good luck!!