I read this sentence in another post:
でも貝のかんじにもうひとつのいみをつけます。
But we will assign another meaning to the kanji 貝.
Putting aside any other problems with this sentence, I feel that もう一つ isn't the right word to use for 'another' in this context. I may of course be wrong, but I would have gone with 別の or 他の instead, but I can't explain why (maybe because I'm talking nonsense).
Anyway, I'm now wondering what the difference is between もうひとつ, 別 and 他 when used to mean 'another'.
I think the difference is subtle here.
もうひとつ means another by way of meaning one other or one more.
別 can mean another by way of meaning different.
他 is probably the truest sense of the word another, but there are cases where you'd use one of the other two in a more natural context.
In the sense of the given phrase, I think all three utterances work equally well:
でも貝のかんじにもうひとつのいみをつけます。
でも貝のかんじに別｛べつ｝のいみをつけます。
でも貝のかんじに他｛ほか｝のいみをつけます。
If もうひとつ was chosen here it was merely because it is probably the first of the three phrases to be introduced in a textbook.