You're objecting to one example of an implementation when you should be focused on the idea. You can clearly define it in such a way as to not rule out multi-party representation as needed. Many other countries have cracked this nut (see Elections Canada). Of course, the history of the US political system seems to be one of NIH...

So don't include them by name. Just say the two major parties. Doesn't matter, doesn't change a thing.

first two parties to finish in the last election would be just as good

Sure. I figure if we ever get to the point where there aren't two major parties, it can either be fixed then, or we're under one party autocratic rule and it won't matter anymore.