Unlikely. Photographers of such class (and their whole team) would not allow the style to be dictated by the preference of an individual retoucher.

You may read hundreds of possible reasons but I doubt anyone here is authorized to speak for Mert and Marcus and their art direction. So if it is really so important to you, perhaps you could try and find a way to contact them directly and ask.

In the forums you will get nothing but guess work. That's why my suggestion.

The "New Yorker" magazine did a feature article a ways back on how Mert and Marcus actually work together on a photo shoot  and especially how they use Photoshop to liberally craft their images and create their signature looks. The article is called  "The Shoot". 

Here are some fun and insightful clips from that article that reveal how they photograph together...  and how they use Photoshop: 

Mert, 33, is Turkish; Marcus, 34, is Welsh. Mert is short and chubby; Marcus is tall and slim. Mert is jovial, with a husky laugh; Marcus is measured, with a mischievous grin. When they work, they take turns with the camera—sometimes snatching it from each other—and although Mert’s taste may incline a bit more toward campy glamour and Marcus’s more toward ironic cool, the results rarely, if ever, betray the dominance of one man’s aesthetic over the other’s. “When you’re looking at their work, you can’t tell which of them had the camera,” Grand said.

Their pictures tend to be luminous, as though the subjects were lit from within, and to feature odd backgrounds, stark contrasts, and rich color. The models’ flesh and hair can appear to be made of plastic.  Many of the images have a staged, formal quality, like the nineteen-thirties Hollywood photographs of George Hurrell,  but with a sheen that suits the video-game age: Jean Harlow meets S1m0ne.   

At times, the women seem almost synthetic, and in some respects they are, because Mert and Marcus do a great deal of post-production work. They are known for manipulating their pictures. They shoot digitally, and then alter the images on a computer screen, using Photoshop to brighten or change colors and contours. They do not hide the fact that they do so. “It’s very fakey, fakey, fakey, but that’s what it’s supposed to be,” Coddington said.  Even real things seem fake: a year ago, they travelled, at great expense, all the way to the desert of Dubai to shoot a campaign for Louis Vuitton. In the end, the dunes looked computer-generated anyway.  “There is a veneer, certainly, of glamour, but, if you look more closely, it is darker than that. There is an artificiality about it.

When a new model, with a new outfit and new makeup and hair, comes on the set, Mert and Marcus take fewer than a dozen shots, because with the digital setup they can see what each one looks like as soon as they take it (the camera, a modified Hasselblad, is connected to the laptop). They take their time making adjustments before capturing each shot. They are a long way from the old stereotype of the fashion photographer as profligate soloist shooting roll after roll, like David Hemmings in “Blow-Up.” Their photographic process more resembles a movie set, or even a football game: Mert or Marcus takes a picture, then everyone huddles around the monitor to look at the result and discuss the next shot, as the assistants run to and fro like water boys.

"Mert began retouching a photograph onscreen of Guinevere gripping a bougainvillea vine, elbows shielding bare breasts.  “What I’m seeing, Marcus: she could get less dark under the eyes.  We get rid of her wig lines.”  He added highlights to the lower lip and rubbed away shadow from beneath the eyes.   “Shall we lighten her skin,  make it more alabaster?” Marcus asked.   Mert tilted his head appraisingly. “She looks good, doesn’t she?   She looks like a strange Hollywood star.  Like Hedy Lamarr or something.”   “We wanted to go back in time,” Marcus explained. “A twisted thirties Hollywood character—Hollywood photography combined with twenties gentlemen’s photography.

These guys make a crazy pair...but like you... I find their work fun to look at and fun to study too.  Their visual imagination is powerful. 

OP... I guess they are are off on a new tangent these days!  Old looks and old styles...they end up looking OLD.  You can never tell where these two visual experimenters will navigate to.  In regards to using a single outside retoucher, the article seems to indicate that they may prefer doing their own thing themselves as they interactively create... making it up as they go along pretty much.   I guess we are seeing a newer, cleaner, commercialized look for a spell.   The flair, the pop, the psychedelic, the plaster, out of bounds looks may have played its hand for a new set of sponsors and audiences.  Keep your eyes open... they are still very good!
The original longer article is still on-line.

The photography market in general has embraced a more natural look, which is also reflected in the retouching. 
As far as M&M's retouchers, they have their own.  You can look up Dreamer Productions on linkedin and see their employees.

Tincture is right. M&M have their own all encompassing production company called Dreamer Productions. They have their own team of retouchers (a couple of seniors, a few mid weights and a couple of juniors) who literally just do M&M work, nothing else. They also hire freelancers (I have been one of them) when they are super busy. They are amazing at colour grading and mainly Mert does all the grading himself.

thanks Nick