How to explain to letter writers that I want to apply to completely different PhD programs? I am not entirely sure yet what I will want to do: It will either by subject A or subject B. Those to subject have very little to do with each other though. (Say one is history of art, the other one political science)

I have decided to apply to programs in both fields and decide later where I would like to go to, depending also on where I would get in.

I am worried, however, what my letter writers are going to think? Is it possible that I will look less determined because of this and consequently get weaker letters? Any advice on how to go about such a situation? Shall I just be completely open about this and tell them that i am unsure?
TALK. TO. THEM. These are presumably former professors and advisors that you know well. Sit with them, skype with them, or buy them a coffee. You might learn a lot about what you actually want to do from the discussions, and they'll surely be happy to help tailor your letters.