Russian dwarf pair: are the fighting or playing?

[floppy91]

Hi I bought my two dwarf hamsters together in early January this year. The pet shop told me they were from the same litter, I'm skeptical because Thunder is brown and Lightning is fluffier, with white with black markings. They live together in the 100cmx50cm Barney Cage. (The pet shop also told me not to put them in this cage because it's too big and they might 'get lost'!).

The cage has two food bowls, two silent spinners, one flying saucer, one sand bath, lots of toys and hiding places, one water bottle.

Thunder and Lightning have occasionally had tiffs, but recently these have become more frequent and more concerning. Yesterday lightning was snoozing on a lava ledge, thunder literally knocked him off it and then they chased each other from one side of cage to another. Twice I have seen them have brief fights where the tumble about quickly (like cats!) . I haven't seen them biting each other and there have been no obvious wounds or signs of ill health.

Both hamsters act like healthy hamsters and run on their wheels, eat, drink, are curious, use their sand bath, look shiny and have bright eyes etc. Usually after a fight they go back to acting normal (I think).

I am struggling to tell if this is a normal tiff or if it's time to separate them. What should I be looking for? How to distinguish if it's serious or not? Anyone been in a similar situation? I will appreciate any feedback!