As few as 1 in 5 Missouri voters could defeat ballot measures under Republican proposal

I agree that sounds wrong, stick to the tried and true, majority of voters to determine the outcome.

Just throwing it out there, but it's not that strange of an idea to use concurrent majority as it is currently used in many aspects of government to protect minority rights against a "populous majority" (i.e., protection against the "Tyranny of the majority").

Our system of the legislative branch is based on a concurrent majority as in a bicameral legislature (such as our House and Senate) you have to have the majority of both houses of Congress approve any legislation (though at the federal level we have a bit higher than "simple majority" in the Senate for most votes unless reconciliation is used).

If you want a U.S. Constitutional Amendment, it requires multiple concurrent majorities - two-thirds of both the House and Senate and then ratified by three-fourths of the State's legislatures (which would require a concurrent majority of the State's House and Senate).

It's frequently used at the local/state level in regards to annexation (you need a majority vote of the "annexor" and the "annexee" to go forward), but this is the first I'm aware of it being used (suggested) at a State level for use in referendums.