>>You shouldn't keep different species together. New animals should also be quarantined for at least 6 months before introduction to the same species. Russians and Greeks have different pathogens that each are used to but the other hasn't been exposed to. That alone can cause huge problems and many trips to the vets. Also if they are 2 males they could fight and do damage to each other. I personally would keep them seperate.

Ditto & more specifically Russians are known to carry a killer herpes virus that can spread to all other species. For that reason they should never be housed with another species.
They are also known for aggression, the Russian can hurt or kill your greek eventually. House them seperately.
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PHRatz