STYLE Element

I've got something very similar in my MS, where one person has taken over the body of another. I do the same, although you could vary it a bit 'said A, or was it B? etc No one has ever had a problem with it.

Maybe you are overstating it? You might only need to do that the once at the start of the sequence and then carry on, or it could get too repeptitive.

I see two separate stylistic tricks: the format and the slashed names. Which is being objected to?

IMHO, use standard dialogue format, but keep the names. Possibly compromise to the extent of using a hyphen in place of the slash. Since the POV char can't tell them apart, show that actually is how he thinks of them, as an interchangeable pair.

Also, "your" editor in which sense? An employee of the publisher who is paying you money for the privilege of publishing your book, or someone you hired to spiff up your book for you?

Here's my issue. You don't need to keep repeating the names, since the reader knows it's one of them. Use the double tag just like a normal tag.

Wrestlers both, their names were Meliuc and Mardeur. They were not related but they looked like peas from the same pod and Pierre could not tell them apart. Both were large men with grey skin and small sad eyes; each had a broken nose and but a few remaining teeth. Pierre found them dull-witted but good natured, and shared pleasant words with them every morning.
Mardeur/Meliuc: “Who goes there?”

Click to expand...

“It is the pot emptier.”
“Ho there, pot emptier.”
Pierre: “Good morrow friends. How goes the pot this day?”
“The pot is full this day.”
“In truth the pot is full to overflowing.”
On rare occasions, the banter would vary:
Mardeur/Meliuc: “The pot is but half full today.”
“In truth the pot is half empty.”
“Either way, your job is half done already, pot emptier.” Accompanied by two identical toothless grins.

Click to expand...