In a word, yes. BUT, I've not seen them in the UK, only in Greek supermarkets (probably in all European supermarkets except the UK TBH!) The only problem I can see is that the combination of 2 leads and the adaptor could pull the adaptor out of that socket.
I would just rotate the 2 toothbrushes on the charger base - they're good for about a week on a single charge, brushing twice a day so shouldn't be too much of a problem.