But note the essential feature:  A debtor can force the acceptance of "legal tender" to discharge his debt ONLY if the debt is denominated in the SAME currency as the "legal tender" money is!  To repeat myself AGAIN:  If you owe me onions (and you could owe me onions!) then I can insist on repayment in onions.   

All that legal tender is, is the ascribing to certain examples (coins, notes) of a particular money a special status.  There is nothing to prevent two different monies being in circulation and each of them having coins & notes which are legal tender.  That this is not common does not mean the contrary.