Are they both females? If they are a boy and a girl, separate them immediately.
If they are the same sex: I know what you mean by wanting to keep them together and only separating as a last resort. I feel exactly the same way about my Robos.
It sounds like the girls (if they are both girls) are maturing sexually and are fighting for their own territory. In my experience, if there is a gash on one hamster, it will only get worse.
"Normal" squabbling among Robos, never involves wounds of any kind and never any verbal noises. You should keep a VERY close eye on them over the next day or two and if there are one or two more bites on the one's bum, then you should separate them. Don't let it get too bad though, since one could get bitten too badly and might need to see the Vet. They fight at night and even though they sleep together in the day, does NOT mean that all is forgiven.
One could still kill the other during the night or chew up ones' bum until it can't defecate anymore.
You can try to give them more space by switching or adding somehow to their enclosure. One common mistake people make is thinking that because Robos are tiny that they only need a small space. However, the opposite is true. They are the tiniest hamsters BUT need the greatest amount of space. Two Robos should be in an aquarium that is AT LEAST 20 gallons. The bigger the better. If they have a huge enclosure you could try creating two areas like putting a wheel, water dish, food bowl in one side of the enclosure and another wheel, water dish, and food bowl in the other side without a physical barrier and see if that makes them happy. Make sure there are lots of solid hiding places for the one to protect herself from the other.
But, you may find that you still have to separate them. I have some of my Robos caged alone because they just couldn't get along with any of their siblings. I find that they do fine alone. I do put their glass aquariums right up against each other so they can see each other. I don't know if that helps or not. Syrian hamsters bond to their owners and respond to being handled and played with but I have not found that with Robos. I pick up my Robos but they never seem to enjoy it or get any "emotional" benefits from being touched by me. I guess that is just their nature.
Just try to keep their life interesting by constantly changing their toys and the position of the things in their cage. They seem to love tubes and digging in deep shavings etc.
Hope this helps. Keep us posted how things are going. I'll check back soon.
Hamstermama