[Glomoro]

I found it hard to tell a harmless squabble apart from a real fight at first myself, but a good rule of thumb is: if it cause any injuries or is severely stressing the either of the hamsters out, I'd separate them and look to the possible causes for a territorial fight before considering a reintroduction. I would not personally use them nesting together as any indication, though. My robos were fighting horribly and still slept together before I could separate them to prevent further issue.

I would generally go for ~360 square inches of floor space for one hamster, which is about what you get for a 20 gallon long aquarium. An equivalent space to either a 30 gallon or a 40 gallon long tank (usually about 36 x 18 L x W, or 648 square inches) would be my butter zone for a pair to live in. Two wheels and two nesting friendly hut type dealies, plus sprinkled food rather than a dish is a recommendation I'd make as well.

Mind, even in an ideal environment you might have one or both that prefer being solitary. As social as the dwarves can be, it does happen.