We want to avoid keeping a couple of Djungarian hamsters to prevent breeding. Is it more natural for the animals to keep just one solitary hamster, or two of the same sex? And if two, males or females? In such a case we'd get siblings to reduce possible initial animosity.
Hamsters are solitary animals and should never be housed together - they will kill and eat one another.
Babies can be housed together temporarily but should be separated when they hit sexual maturity (~4-5 weeks).
There is the odd case where some hamsters will get along as adults however they tend to one day change their minds and kill their cage-mate. I've seen this with many different species of hamster, including dwarf - it's pretty mortifying, especially for children.