Sometimes a two-word tag may describe a meaning better and in the beginning I was using them like this "two words" but I realized that it doesn't pass html validation, so I put a dash between like this "two-words".

Unfortunately, K2 automatically removes the dash and make it one word like this "twowords".
How to resolve this problem?

The dash is invalid for the url of the tag. Try to use the underscore (_) instead, to separate the two words.