This is the same dilemma as you get on a plane, and there is no official answer. The logical thing is for everyone to take the arm rest on their own right hand side, as that is more comfortable for most people. It's a bit unfair to left-handers (like me, for example) but we are used to using right-handed stuff so that really is the simplest solution. And everybody gets something to lean on.

However, for this to work everybody has to be very fair-minded, logical and well organized. And while you could find yourself sitting in a whole row of fair and polite people, I can only say that it has never happened to me! What tends to happen is that there are two kinds of people - those who take both armrests and those who end up with neither. Sadly I seem to belong to the second type. Which are you?