Personal statement suited to both History and Russian AND Economics and Russian

The general advice is: don't. You need to show a deep level of interest in the subject you're applying for in your PS, and failing to do so is like to lead to the word 'unsuccessful' showing up on Track. For some subjects, it's possible to cover things that apply to both (e.g. politics and international relations), but history and economics are too different for that to be feasible.

You're going to have to choose sooner or later, so I would suggest that you should choose now so that you increase your chances of getting an offer.