I do ancient and history extension. However, I know my friend last year did: Ancient, Modern and History Extension.
You need 2 things to be successful in history:
- Content - and a lot of it
- You need to write well, know how to answer the question
Modern History in my opinion looks a lot more interesting, it scales better and I think tbh easier because it concentrates on argument more than content.
I'll be honest though, at one stage I as overwhelmed with the amount of content there was in Ancient, but if you work hard, learn the content - you'll do exceptionally fine.
All the best. I highly recommend doing both, it's very enjoyable and you'll do well if you learn the content, go over the content consistently and you know how to answer the question, incorporating sources etc.
Don't be worried about the work load, because think of it as another 2 unit course, if you were doing legal studies in place of modern your question wouldn't be any different.