Hi, I'm sorry to hear that! It's a shame your first post is a sad one

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It's hard to say why she might have died, dehydration, too much citrus, something in the diet that didn't agree with her can all cause runny poos, there are loads of reasons. Does the other have runny poo or are her's normal?
She could have gone in to a state of torpor which in the wild in a large colony isn't a problem as they have others to keep them warm, if it's not caught soon enough in our captive gliders it can end in death. They literally just go to sleep and the blood sugars drop too low. This usually happens if it's too cold in the room especially if she hadn't eaten that night or are under the weather.
For information on intros have a look here -
http://www.sugar-glider.co.uk/forum/index.php?topic=69.0I personally think 3 is the way forward, as social animals they don't like to be alone, if the worst happens and one of the trio dies you are still left with two. So yeah, go for it, get two more. You can introduce females or neutered males if you don't want to breed

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What are you feeding them BTW?