I don't think GG would go any faster. We just wouldnt have Nezumicon at all. If I had to guess, the production speed is mostly limited on the art side rather than the programming/writing side. That's why the games have two different artists, but they share the same writer. The devs have decided for one reason or another that it is best to have one artist per game. I say this as a solo game dev who doesn't do VNs, so take everything I say with a grain of salt. I do all my own programming and art, but for an RPG.
There are two big reasons they would have chosen to have split games, given the art bottleneck. First, I wouldnt want to match my art style to that of a second artist, its why lots of indie games in small studios have only one person doing art. Doing so would slow down my process immensely, and its probably the same for the two game artists. If I were in this art bottleneck situation, I'd choose two games 10/10 times before I'd put a second artist on the main game.
The second (and probably more important) reason is money.
I'm not a financial analyst by any means, but I don't think putting both artists on one game makes sense for them, even if they could keep the same number of total route updates. They likely couldn't charge much more for GG even if they gave it the additional time and updates taken for Nezumicon. Frankly, the amount they charge per month is already kinda ridiculous for the amount of playtime added in each update. That's why we're all here on the piracy site instead of paying for it. They'd be doing more work for less money per team member by focusing on the single subscription for GG and adding an artist.
As for why there are never Nezumicon sponsors, I'd be willing to bet that it's just because any of the whales would prefer to shell out that money for the game that is further along with a character they have had more time to grow attached to. I'm unlikely to sub for more than the cheapest access for one month. If I WERE to burn some money I don't have on an update, it would unquestionably go to advancing the plot of my favorite GG character, rather than my favorite Nezumicon character. Purely because the GG story is to the point it's actually getting places.
I'm gonna ignore most of your post, because the points about "why" they're doing it the way they are are largely irrelevant- we all know the current model is because of constant money. They're not as shitty about it as Yandere Dev, but it follows the same cancerous crowdfunded model which rewards putting out a trickle of content to keep people thirsting for more, and paying $200 a pop just to see their preferred .png have more content.
Now for your point on art, I'm too lazy to put pictures together, but I don't feel the two artists are *that* different from each other- I imagine even people who aren't explicitly aware the game's made by the same guy would have an inkling if they've played both, and likely wouldn't know it is two separate artists.
Furthermore, games having multiple artists working in tandem is not anything revolutionary- a game called Youmaen had 10+ different artists, with all the girls in it possessing extremely different styles as a result (nowhere close to the similarity GG and Nezumicon have) and yet it works regardless. One artist could be assigned to one set of giantesses, say Timfalme, Heaven and Hell kingdoms, and the other could be assigned to Viridice and Lycolis (that's just an example, I'm aware there's a huge imbalance towards the former category).
Technically speaking though, it may actually net them more money if they did focus solely on GG- it would mean more sponsored updates, and Nezumicon's likely not selling all that well either, evidenced both by the lack of sponsors as well as how long it can take for us to receive an update for it here, compared to GG. Even if they didn't make more money though, it would still be more ethical business-wise. I'll give them props, they are consistent- I don't think they've ever skipped a week due to sickness or vacation, and that's impressive, but everyone and their dog knows that at this pace, GG aint never fucking finishing.
