I have recently graduated having done both. I loved it, in year 10 I was on the brink of choosing geography instead of ancient for similar reasons that you express, it just didn't sound good to do two histories. It felt too narrow. but I'm really glad I did. One thing people miss in their responses to this thread is that doing both histories is not like doing lots of the same subject -- ancient and modern at this level are completely different disciplines. Ancient felt much more hands on to me, you can't just read a textbook account. Instead you're obliged to get stuck into primary sources. Because you're dealing with happenings of so long ago, everything needs to be supported by an ancient written/archaeological sources and you start piecing together your own narrative. whereas in modern, we have the facts (dates, names, places) in minute detail. So it's less dominated by primary sources, it's much more of just memorising an established narrative. So I found it less interesting, more rote-learning involved.
But it didn't get monotonous, it didn't feel like doing history on history (and I even had the same teacher for both) because they are very distinct subjects. Advice on doing well: always interpret passages/quotes that you take from a primary source, don't just present them. Draw specific insights from the evidence you bring into an essay. And you don't need to memorise heaps of quotes, marks are also awarded for paraphrasing
