In the year 2000, Belgium and the Netherlands became the first countries to cohost a major, FIFA-sanctioned football tournament when they staged the 2000 European Championship finals. It was an all-around success and pointed the way forward for other cohosted tournaments.
Except, perhaps, the 2002 World Cup.
While other countries and regions (Austria and Hungary; Norway, Denmark and Sweden; even Greece and Turkey) have made, or have been considering making, joint bids for major football tournaments, the one big difference with the Japan-South Korea conjoining is that these bids were joint bids from the beginning. In such cases, essential areas of dispute -- who gets what and where -- are taken care of even before the bidding gets under way.