Good point, I'll try and explain it the best I can, as it can be quite subjective. If a track has entirely different routes for the two cars, it won't be disqualified from the MTC, but it may be marked down for its theme, as the idea is to have the route as similar as possible between the two cars. In fact, I will add that to the list of tips.
For simplicity's sake, even if OpenPlanet is released very soon, the current rules will still apply.
It's been discussed whether or not to have both tracks submitted, and having just a single track erases the need to check the entire track between both versions, and I've been told that changing the driving car only wouldn't change the map UID, so it wouldn't really work to upload anyway.