I've seen this thrown around by Pies alot for a while, and I'd just like to bring it up again.
First of all, Why?
Let's look at the comparison between Gunner and Pilot. Both of these classes lack heavily in two areas, their common ground being the lack of engineering tools.
Captain/Pilot:
Captain's tools tend to be irrelevant most of the time and only comes into play when a component/components of value are damaged and it becomes ultimately pointless to keep piloting for a short amount of time. Even so, some ships' layouts don't permit this style to an extent at all (Squid, Goldfish, Mobula). Couple that with pilot's array of tools that benefit their ship in some way or other, I'd say pilot meta wouldnt really change even if you took their engineering slot away completely.
Gunner:
This is where the problem starts. What happens when a class that is effective as long as it's functioning as intended in a specific position, (Think pilot on the helm), gets forced out of that position in some way or another? I'm looking at you, Artemis and Flamethrower. While I don't have many problems with these two weapons and anything of similiar nature, I feel that gunner can easily become redundant as soon as they, and other disabler weapons come into play. What if there was an item or a function in the game that forced the pilot out of his helm? Fire stacks maybe? Engineer would end up reigning supreme. Gunner's effectiveness relies completely on a specific, interactable and destroyable component type that the engineer's playstyle encompasses in both maintaining and in some cases, using aswell. Unlike the pilot, who helps the ship function from a specific, undestroyable position that he can go as far as to force his crew out of should they take it from him. Furthermore, no matter where you look at it, gunner is considered "crew" and therefore has much more freedom to move about and interact with the ship than the pilot. And in some cases, he HAS to do this. This is part of the reason why competitive teams run triple engineers incredibly often. They simply lack the reactive power of a mallet-spanner or even pipewrench-chemspray. Ultimately, Pilot is countered by disabling 4-5 components (Engines and balloon), while gunner is, in most cases, countered by disabling 1 component due to the nature of how crew positions have evolved throughout the game. This is also another strong point of the engineer - It has no hard counters. The rest of the classes are countered by disabling components that allow them to function effectively- Engineer's literal job is to deal with those disables. If anything, they -counter- disables. Fire, Shatter, you name it.
So what would happen if gunner was given two engineer slots?
Two gunner builds would be viable. Period. Scary, isn't it? Well no. Let's imagine your typical metamidion being maximized to it's effectiveness with 2 gunners:
Top Right Gatling: Lesmok, Greased, Heatsink. Kit: Pipe wrench, Buff hammer. For once, gunner is almost exactly equal to, if not superior to, a buff engineer on something that isnt mines or lumberjack. This guy can do anything a buff engineer can do, with the added bonus of having an ammo handy for any range. The only thing it falls short at is dealing with fire, which it can do anyway but at the slight cost of a decrease in range/damage due to how Heatsink works. But they can literally get around the damage part completely simply by having the buff hammer around. They would also be responsible for all buffs aboard the ship as the most optimal two gunner build I can think of would involve only this crewmember carrying it. Though that would mean the gatling would fire much later if they don't predict combat or spend too much time on the lower deck, which is less of a hassle when you're on the mortar, which needs to standby for some time before it starts becoming amazingly effective. Come to think of it, this build could be better with Mortar-right and Gat Left, but let's continue anyway.
Top Mortar: Same build Lesmok, Greased, Heatsink/Charged?/Incendiary?/SomeOtherAmmotype. Kit: Pipe Wrench, Chem/Ext. Suddenly, the mortar has become much, much more versatile. Not only will you able to unload on enemies with greased as usual, but you'll also be able to greatly extend your effective range and have an easier time chasing faster ships as if you were using the old Light Flak. And ontop of this versatility, you're also able to maintain your gun, the balloon, and potentially the entire ship against fires if the need arises.
Other situations include:
Galleon lower deck, Mallet-Spanner. In the regular 2-engi-1-gunner format, this would be nothing but a great buff to the lovely, situational and easily countered bulky mass that is the galleon.
Mobula: Top gun, Pipe Wrench-Buff. Since it's apparent that the Captain can help the gunner extinguish his gun, this would basically be a buff engineer that can control his range. A bit OP? I do agree that it's superior to buff engi top in almost every way, but gunner ammo types arent as powerful as the current buff engineer's fire control ability + damage boost on guns to begin with. Furthermore, this build would also fall short on assisting your engineers against fire damage, something you cant actually deal with without the pilot's support, thus further contributing to Mobula's vulnerability against this specific damage type.
Goldfish: front gun, Pipe-Buff/Mallet-Spanner: This would be a fair buff to the Goldfish aswell, and would also extend the meta, allowing the choice between damage/ship performance boost versus disable protection.
Spire: Lower deck heavy gun, Pipe Wrench/Chem: This in particular would work VERY well with spire builds that encourage double engineer top and leave the gunner alone downstairs, as not only will he be able to take ammo types for two different guns to maximize his effectiveness in regards to gun versatility, but you'll also be able to perform chem cycles on the lower deck without one of the engineers having to jump down and decrease top deck's survivability. I could see this working very well with the hwacha's reload time, but it could still be a bit cumbersome to perform in combat.
So would gunners be OP now? Well, perhaps it might seem a bit powerful in comparison to Double engis-one gunner format, but for instance, our first example of a metamidion completely fails to exceed past the power of a triple engineer metamidion, which would basically have on top, a mallet-spanner gunner with the added bonus of a buff hammer + a buff engi near balloon, which makes it much easier to get the damage boost on both of your guns at the expense of controlling range.
Would the gunner meta that follows after end up outclassing Double-Engi-One-Gunner setups? I dont know, maybe. But I'd rather have double gunners stand up as a fair sidegrade to Triple Engis than for muse to discriminate gunner gameplay wise any further.
I think I'm a bit done for now. Discuss, if I haven't covered literally everything already.