The Duna would likely be a little small for two robos together. It could be a number of things, it could also be absolutely nothing. She might be feeling a tiny bit off colour (nothing serious as she's eating, drinking, playing ect) and not wanting to interact of come out. She might have hurt herself and again not want to be bothered.
If the separation was a while ago it's unlikely she is suddenly feeling lonely. How long has her behavior changed for now? If it's not long, see if she settles again, as it's likely to be temporary. Dasher has days where he's in a total grump, and doesn't want to know, then most days he's his normal self.
I wouldn't rush into getting another ham just because she's a little unsettled.
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