I would have thought the question is which do YOU want to keep?
They have different personalities and being different sizes need different toys etc
And there is no risk of babies if they are different species and not genetically compatible....however I have noted that it looks as if the male has a very slight stripe down his back and could be another dwarf ....the photos are really not clear enough to determine. He also looks very young to have been re-homed.
The main issue is as the others have said, if different species, they will fight and this could lead to serious injury or death so I would make that decision as soon as you can.
It was very naughty of the store to sell you the two hamsters without being more helpful about what they were, how to house them etc so I hope they are prepared to take one back.
