I have a robo and also wanted to get a pair but working out the set up was going to be fiddly and I also wasn't sure which cage. So I went with getting a single one and setting it up how I liked, for variety and enrichment. And as you know pairs often end up having to be separated. Half a detolf might be alright for a single roborovski but it can depend on the hamster. The other thing is that partitioning a detolf for two hamsters doesn't always work well - it's common for one to somehow get past the partition.
If you had two and needed to separate them you could leave one in the detolf and have a bin cage for the other one maybe.
I think Erin's Hamsters had a detolf for her dwarf hamster (can't remember the name of the hamster
. If you had a deep digging area, a flying saucer at that end and the house at the opposite end they would probably use all the space
Although our robo ignores houses and digs holes and makes a burrow near his flying saucer. He also has a wheel and uses both.
Difficult decisions. I think if you want a pair, then get a pair, especially if it's robos - but be prepared to need a separate bin cage maybe. It's not impossible to divide a detolf but would need to be done very well so there's no way one could get through to the other side. I did read a sad story where one had chewed through and killed the other one.