I noticed that 「もう」 can mean both "already" and "additional", such as the following sentence:
Can mean either one of:
- I already drank two glasses.
- I drank additional two glasses.
How can I differentiate between the two meanings?
I guess that the most reliable way is decide from the context. But at least in the Tokyo dialect and other dialects with the same accent pattern, they have different accents.
If you really want to distinguish them, you can emphasize the "already" one with すでに （既に）.
I can see what you're thinking, but the second English meaning does not arise from the Japanese sentence provided.
もう二本飲みましたよ。 Yep, this means "I already drank two glasses"
もう二本飲みますよ。 In present/future tense it means "I will drink two additional glasses"
But by saying もう二本飲みました, it does not become "I drank two additional glasses". To say that you might say 追加として二本飲みました.
Regarding pronunciation guides, from experience this can be less reliable than context, since even native speakers regularly mix them up.