Hi and welcome. Sorry to hear about Hollie and Fearne.
Unfortunatley is it not unusual. If thery have always seemed happy together and not fought it is likely that one died of an unknown condition then the reminaing hamster ate the body, they do this in the wild so that preditors are not attracted, as sad and distressing as it is, it is a natural instinct. The other will have died as a result of injesting the dead one, it can make hamsters very poorly when they do this.
This type of behaviour also happs with litters. If mum suspects something is wrong with one of the pups or a pup dies she will injest the pup to keep her and her young safe. It is compeltely natural, although a little disconcerting for us humans!
Ave and Mia sound gorgoeus though so a big welcome to them and you. I t would be lovely to see some photos of them it you have any