Multiple parent categories

I see this is quite a common need, but without any answer.

I need to setup more parent categories to one category. I know that it is not possible using standard functions, I’m quite a skilled developer but i do not see also any way how to create such plugin (only with creating completely different taxonomy).

Does anyone knows any easier workaround on sollution? e.g. with combining tags, but there must be some definited relationship between parents and childs (N:N).