I am not sure it matters whether it's a sister or brother pair regarding fighting breaking out. Generally on here people recommend not keeping dwarf hamster pairs unless you're experienced. But - I am aware that there is an article on the National Hamster Council that does recommend robos living in pairs. You would still have to take all the precautions. ie at least 2 of everything (more than 2 is ok as well) - houses, wheels, food bowls, water bottles. I'd suggest two wheels and two flying saucers. One wheel big enough for them both to comfortably run in together - have seen it suggested that a syrian wheel works well with a pair, with the second wheel being a smaller one. So either two 8 or 9" wheels or an 8" wheel and an 11" wheel that spins really easily and has a bit more depth maybe. Easier to use two 8 to 9" wheels which are a good size anyway.
And no levels. No shelves. Tubes or tunnels need to be a good size so both can run through easily without one being able to block off or guard an entrance. With anything that is a level it can be like a new territory and one can claim it and see the other one off.
Houses need at least two entrances - multiple entrances is good. Again so one can't block the other one in.
Cage size views vary. Some say if it's too big fighting may break out, others say if it's too small the same thing - so somewhere in the middle maybe
Oak Farm (Roborovski specialist) on their webpage reckon something like a four foot tank for a group of robos. I would say 90cm would be ok for a pair and am aware of the 90cm by 45cm Exo Terra terrarium being used for a pair. An 80 x 50 cage with narrow bars would be ok as well. But I think 90cm or 100cm might be good to be able to fit all the two or three of everything in. It is possible in 80cm though and still have plenty of floorspace.
I really wanted to try this - and was going to adopt a pair from an unexpected litter. But there was only one left in the end so that made the decision. Before I knew that I had been finding it quite hard to work out a set up for a pair, and it was so much easier setting it up for a single robo.
But I'd say if you want to, then try getting a pair. I don't think it matters if it's boys or girls really as long as they're siblings. If your careful you may be lucky and them live together for life, but you need to keep an eye on things and always be prepared to separate and have another cage to hand in case of an emergency (or a bin cage ready eg).
They can often live together happily for some time and then squabbling break out leading to fighting - when they are older and hormonal - but not always and cage set up is a big factor. Even to the point of not taking one out of the cage on its own. In that short period the other one can get territorial about the cage and that could lead to them falling out.
It isn't just fighting, you need to keep an eye on dominance and bullying as well. If one gets bigger and seems more dominant, they could be hogging all the food. A good idea is to scatter feed either instead of or as well as, two separate food bowls.
There aren't many safe cages for young robos. The height needs to be reasonably low and the bar spacing small enough they couldn't try and squeeze through and get stuck. I have the Savic Mickey 2XL cage (called the Lixit Mickey 2XL cage in the US) - which is basically the same cage as the Hamster Heaven but has 7mm mouse bar spacing. Sold as a mouse cage. But it's quite expensive new, especially in the US. I do sometimes wish it was a little bit bigger for our robo. Otherwise, tanks work well. The EXo terra mentioned above seemed a nice option but it is also very expensive - only real difference between that and a tank is it has front opening doors.
A 40 gallon breeder tank in the US is about 90cm by 45cm so that might be a good option if it is quite full with items.
Robo Pairs
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