I think I'd be happier with the first sentence if it was written as:
If you could increase your number of hours to 48/week, it would help you become more fluent.
It maintains the same set of auxilliary verbs in both halves of the sentence. It is also a bit passive, compared with the more active second sentence:
If you can increase your number of hours to 48/week, it will help you become a more fluent speaker.
I'd choose between them on the basis of my expectation of the increase happening; the first has a resigned air to it (if you could, you would, but you probably can't, so you won't), whereas the second is more positive (if you can, you will, and since you can, you will improve) or authoritative (an order disguised as a suggestion).