Cristiano Ronaldo and Michael Jordan -- two of the wealthiest athletes in the history of the planet -- doubled up for a night out in the French Riviera ... because this is what extremely rich people do. 

Ronaldo has been yachting it up with his longtime girlfriend, Georgina Rodriguez, and their kids -- but clearly, they got a babysitter and found some time for a night out with MJ and his wife.

Unclear where they went ... but with Ronaldo reportedly worth somewhere in the ballpark of a billion dollars ... and Jordan reportedly ALREADY in the tres commas club -- they can afford to eat anywhere they want! 

Remember, Ronaldo has been sailing around on a $200,000-per-week yacht -- that comes with its own gym space and MOVIE THEATER! 

Over the years, Jordan has been spotted floating around on expensive boats.