1. No. Businesses (by federal law), aren't even required to accept dollars now. 2. Yes, but would you use Honest Achmed's Used Cars and Dollars? 3. (Private)

1. No. Businesses (by federal law), aren't even required to accept dollars now. 2. Yes, but would you use Honest Achmed's Used Cars and Dollars? 3. (Private) Ratings Agencies would rate currencies and maybe each other. The market picks the winner based on trust, convenience, or whatever else it cares about. In practice, I'd expect an 80/20 rule. Two currencies would make up 80% of all transactions in a given region, with a rather large tail (gold/silver/basket of commodities backing, fixed exchange rates, etc). Multiple currencies is nearly unworkable with paper money, but quite trivial with digital paper money.