[development] Parallel Vocabularies

A discussion about a feature in the Glossary module has raised an interesting idea, which I'd like to have some comments on. In a few cases, there may be a need to have a "parallel vocabulary" that remains synchronized with its twin. In the case that brought this up (Glossary), one might want to tag nodes with one twin, while the other is used for more descriptive uses (such as flagging glossary terms). I don't think the code would be all that difficult to do, but I was wondering if anyone else felt there was a need for something like this, and if so, could you detail a use case that could help "sell" the module and encourage someone (me) to write it? Nancy E. Wichmann, PMP