I don't know about countries (maybe there are some indeed) where more than one currency was declared as legal tender nowadays. If we proceed from your assumption that some state would establish bitcoin as legal tender, we might as well expect establishing bitcoin instead of its today's legal tender (fiat currency), right? If you assume that some government would set bitcoin as second legal tender on par with fiat currency, then Gresham's law would inevitably kick in, so why would they?

Or, are you misusing the term legal tender too?