I'll stay here for two more months means that I have stayed 'here' for at least one month. I'll stay here for another two months means that I have stayed 'here' for at least two months. I'll stay here (for) two months more is possible even if I have stayed here for less than a month.
Top answer
Your third sentence is the only one with a "guaranteed" meaning.
— Philip
Your third sentence is the only one with a "guaranteed" meaning.
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