Making two countries to host the world cup is a bit very rare to me because I don't think this had been happening before. What I know is that it's only one country that normally host the world cup. Now it would involve two countries.
We had the world cup with two hosts before if I'm not wrong but still, we never had three countries in a world cup however America is the primary host and Canada/ Mexico are just helping America but I think the reason of why FIFA made three countries hosting the world cup was increasing the number of teams in the world cup that's why we need more hosts instead of one country hosting the world cup.