Red and Yellow
"They do exist!" - Red and Yellow are a strange sort of mix, with some elements of NASB Ren & Stimpy, but also an Ice Climber-esque gimmick. It's pretty easy to split them up and because of this their moves have different uses based on whether the M&Ms are together or apart. Red leads, Yellow follows.
Light: Red flicks his fingers to the tune of a song Yellow plays on Blue's saxophone as a multihit - when apart Red's flicks are faster but deal less damage due to a lack of rhythm. Yellow doesn't have a solo equivalent to this, but Yellow can perform any other move or action while it's being used, making it good for desyncing the two
Light Dash: Red rolls while Yellow walks on him, vulnerable. When apart Yellow also rolls and isn't vulnerable, but the split moves don't go as far or fast.
Up Light: Red takes off his shell and swings it around - Yellow responds to this with disgust if together, but when apart he's also willing to be a bit of a party animal and do the same.
Down Light: a sweep kick performed by both - if together, Yellow will kick Red by accident and send him flying a bit forwards, dealing damage in his path. This will seperate if Red is kicked off a platform, but otherwise it isn's a seperating move
Air Light: Red spins his hand around his body and creates a trail of Minis. When seperated both will do the move, but won't create the Minis and therefore go without a disjoint
Up Air Light: Red presents an inanimate M&M. When alone Red and Yellow both present half-eaten M&Ms, which have worse range but are more powerful thanks to their shap bite edges
Down Air Light: Red does a downwards punch with Yellow keeping ahold of him. When seperated, this move makes Red fall but not Yellow.
Strong: Red slams a bowl of M&Ms into a foe's face - when seperated Yellow does the same with no differences
Strong Dash: Red does a running uppercut - with Yellow, he'll hold Red and rise him upwards, making the move slower but with a better upwards range. Alone, both do the same, faster, uppercut that sticks to the ground
Up Strong: Based on the infamously defiled "We're Putting Caramel Inside M&M's" ad, Red splits up a caramel M&M upwards and downwards, this sends foes upwards. If together Red will end the move short because Yellow tries to lick the caramel, but if alone both will do the move - Yellow licks his solo caramel which gives the move some extra saliva-power but makes it last shorter and gives it shorter range, while Red's is out longer, has a longer vertical range, has roughly the same power as default, and has a shorter endlag than the other two versions
Down Strong: Red slams Yellow into the ground like Ren and Stimpy's down strong in NASB - if alone Red just uses a giant bag of regular, inanimate peanut M&Ms, which is super weak but has astonishing range + disjoint, while Yellow falls over which is stronger than the together move but also has worse range.
Air Strong: Red retreats into a "primal" form so to speak and spins a la Pac-Man - Yellow does the same but instead of spinning he just tilts left and right because of his vertical shape. He goes behind Red to give him some extra range when together, while when alone he only deals damage at his tip
Up Air Strong: Red does a flip kick upwards with Yellow doing a punch downwards, hanging on to Red. When alone Yellow does an uppercut for this move. When alone, they're more vulnerable at the bottom when doing this move
Down Air Strong: Red does a downwards stomp. With Yellow this will send Yellow falling downwards, making this a seperater move, while when seperated both will stomp downwards - Red spikes, Yellow just propels downwards
Special: Peanut Shield: When together, Red holds Yellow as a "meat shield", with Yellow following up with a punch as a counter. When seperated, Red does a command grab shake attack while Yellow does a slower, non-counter version of his counter punch
Side Special: Seperating Kick/Spokescandy Alert: Red will pick up and kick Yellow to seperate him, the trajectory being aimable and Yellow dealing damage in his path. If they're seperated, Red will call Yellow to him with a whistle. The whistle can deal damage but no knockback, and gives Red super armour but Yellow none. If Yellow is hit while returning the move will be cancelled.
Up Special: Diving Board: Based on the Chocolate Pool ads, the M&Ms jump off a diving board and "cannonball" downwards as an attack if together. If apart the move is much quicker and goes higher but doesn't deal any damage.