When 2 or more characters speak in unison you can generally do something like this
"awwwww, she's so cute!" the girls said in unison in near perfect harmony
but what about when the line is only slightly different
Person 1: but i'm not...
Person 2: but she's not...
How would this be written?
I've seen it written on the same line in defiance of the "new speaker, new paragraph" rule:
"And is this your girlfriend?" Mom asked.
"No, I'm not — " "Absolutely not — " we both protested immediately.
1. Generalise
I ignored their spluttered protests
This works fine when it doesn't really matter what they're saying.
2. Share a dialogue line
"I said, Do you get me?"
"Sir, yes, sir!" they shouted in unison.
3. If the meaning of each is necessary, call it out.*
Use separate dialogue lines, but indicate in the narration that they spoke at the same time.
"Are you two married?"
"Of course not!" I said.
"Yes, for ages!" Marie said at the same time. A puzzled frown creased the big man's brow.
4. Try to avoid this construction
Really, as others have said, this is a spoken word trope. It rarely or never happens in real life, so there's no need to try to pull it into written dialogue either.
You don't. That is a TV thing. The page is not the screen.
How you tell a story in each medium is an artifice. You are never reproducing all the elements of real conversation, all the halts and tics and repetitions, and all the banalities of everyday speech would be catastrophically boring on the page or on screen. So you create a stylized dialogue, and you use the stylistic tropes of the medium you are writing for. People interrupting and talking over each other works on the screen. It does not work on the page. It just requires too much stage direction, which distracts from the dialogue itself, and the reader does not experience the dialogue as overlapping because on the page the reader can only receive a single stream of words at a time. You just can't reproduce the effect that this would have on screen.
So, you stylize the conversation in some other way for the page. This can include dialogues that the reader can read but would be very difficult for an actor to say. (Harrison Ford is famously supposed to have complained to George Lucas "You can write this s***, but you can't say this s***" (or words to this effect).) The screen is not the page.
So come up with an alternate approach to the dialogue that gets the same message across about character and motivation without having people talk over each other. There are always multiple ways to get the character of a conversation across. You need to choose one that works best in your chosen medium.
It's not always necessary to actually write down exactly what they're saying. It might be simpler to simply describe that they spoke at the same time, and their general tone, for example:
They both blurted out their protestations, each of them trying to be the one to deny first.
This can then be followed by each of them saying their own line of dialogue, once it has been established that they're trying to say it at the same time, although it may not be required.
I have been struggling with the same issue, and I have found that the best way to handle dual dialogue that is different by writing the main character's line first, then the other character, underneath. Like this:
“So, why are you here all by yourself?”
"My father had one of his associates drive me." I said hastily.
"My father had an associate bring me." she said at the same time. We glanced at each other awkwardly.
