I do not currently have access to RPG Maker MV, but I'll try to help you the best I can.
How I would do this, is I'd make a skill that does absolutely nothing except call a common event. (Only because common events happen after the actual attack) You can call a Common Event through a skill in the Effects section.
For this example, we'll call the common event "Team Attack" and I'm going to use the names Diego and Terry for this example.
For now, I'm just going to assume you know how to disable the skill when one of the actors are out of the party. If you don't, then tell me (and specify certain things like if the player can willingly get them out of the party or not) and I'll get back to you on that.
In Team Attack, have it set Variable (X) to Actor Terry's MAT. (You can do this by going to Change Variables...)
Then, have it Force Action on Diego to make him use this other skill I'll just call "Teamwork!".
(By the way, Force Action doesn't let you choose the target, but I believe that somehow you can make it so that a skill tries to target someone with a specified state if the target is randomized. I can't remember how, but I'll look into it when I can.)
(Force Action lets you target the previous target, so just select that one and you'd probably be fine.)
I believe Force Action works by indexes, if I remember correctly. So for now, all I can think of for using Diego on the Force Action command is using a list of conditional branches that test which party member slot he's in, which I could probably explain more in depth when I can use MV again.
But anyway, you could use Variable (X) in the skill "Teamwork!" so that it modifies its damage.
Here's an example for the skill formula:
v[x] + ( a.atk * 2 )
Of course, instead of using v[x], you'd have to replace x with whatever variable you decide to use. If you need anything specified a bit more, feel free to ask.
Hope that was helpful!
Also...
@Aethers
I'm actually not that good at coding at all, or at least creating it.
However, in my quest to help you with my current wisdom and knowledge, I thought what you might want was a plug-in that added more parameters (that you'd be able to set/modify through note tags). I know that I, personally, would LOVE to have a plug-in like that, but I sadly couldn't find anything. But then again, I was always the worst at researching...
If you can somehow figure out how to use something like that, or find a plug-in that would work that I obliviously missed, I'd appreciate it if you could notify me, as I'd like to use something like that as well.