from Wikipedia: “The two 'M's represent the names of Forrest E. Mars Sr., the founder of Newark Company, and Bruce Murrie, son of Hershey Chocolate's president William F. R. Murrie, who had a 20 percent share in the product.”

On the decoding symbols homepage, the author warns in the description for the article Squid Game, Venus, Mars, E.T. that it’s very long. But here’s the part about M&Ms:

It’s notable that Hershey and Mars Inc. despite being the big Candy competitors would get together to create the most popular candy in the world: M&M’s. This is why M&M is likely a comm for cooperation between “Shells” (proxies acting for another)- and this is why most of the ads for it focus on different types of M’s working together. Consider the symbolism of the “M” a mark of ownership in relation to a “Shell” (I’ll return to this point later) Mars grew beyond Hershey and bought out their rights, they fully own M&M’s now, but one of the M’s will always represent Hershey.