If you expected porn, you’re wrong here. 
Hey folks. If any artist or any person who worked with some artists on a project could give some tips, I would really appreciate it!
So, here’s the thing.
For now we were two people. Me, the programmer, and the artist. Recently we got help on the game-art by another artist. She’s doing a great job but we talked with her about matching her artstyle with the “first/main”-artist.
How’s your experience? What’s the best approach to match artstyles?
Easy answer: it cant be done fast.
Artstyle is the same as personality, genes, DNA and fingerprints. For her to “match” ur artstyle will take her a long time to do unless u use a very simple one. There will always be unique strokes and expressions in art, cloning it to someone elses is not something i would have easy time doing, and im fairly good at adapting my art towards something.
Can we have some examples of her art as it stand now, and then the target art she needs to learn? Would be very easy for me to see if she could do that in a reasonable timeframe.
Speaking as an artist, this is an excellent answer. My style is the sum of all my experience, and that includes years of “Oooh, I love this artist, I will try to draw like them!” Ultimately, my art style still has traces of those emulations in them and so that makes… my own style. I have taken those emulations and made them my own but they are still the sum of all the parts.
Likewise, telling an artist to adapt their style to someone else’s isn’t going to be easy. They can do it… it will take time and many frustrating attempts, usually.
I guess for an independent artist this would be extremely difficult. But if an artist learned the fundamentals it shouldn’t be that hard. What you do is make a design guide for the artists to follow, by breaking it down this way any skilled artists should be able to work well together and have the hard mesh well.
Design can be broken down by workflow, technical limitations, hard shadows, soft shadows, breakdown of the anatomy of eye stylization, if there is rim lighting, color range, proportions and basically anything else that stands out for that style. Break it down into rules for artists to follow and it shouldn’t be a problem.
Furthermore artists don’t just have 1 style and only work in that style, there shouldn’t be any time required to really adapt. It’s not like your relearning how to draw from the ground up, your just using what any artist should be learning to begin with in a different way whether it be simplifying shapes and forms or just using different tools to do that same thing you already would be doing in the first place.
Maybe looking in information on Visual Style Guides could be helpful.
I absolutely loved kelde’s response. Like mentioned above, every artist has their own style. Although it can be done with time and practice it can be difficult for the artist to move out from their artstyle and copy someone elses artstyle. Speaking from personal experience I always tried to create my character models with the same art style from games such as super smash brothers and old ps2 titles but ended up coming up with my own unique artstyle for my character models.
Beardbotnik also has an excellent response as well and everything he said is right. For independent artists (like myself and many other people on the unity forums) it might be more challenging depending on their background in art, but for a professionally trained artist, like the kind of person that worked in studios, learned studio training in school, etc. It should be no problem to easily adapt to the art style. Because like Beardbotnik said, an artist does not have 1 single style, they should be able to work with multiple styles and easily adapt to them with little time if they apply everything they learned.
This is just thought but although it makes more sense for the game to have 1 constant artstyle throughout, I think it would be really cool if you incorporate both artstyles. Let one artist do his style and the other artist do her style and put them both in the game. The game would be unique and people would be able to tell who worked on the game by noticing the different styles. Again just a thought I think would be cool. 
Damn not what I was hoping for…
You need someone to be the production designer and that person will create a style guide. The style guide breaks things down into rules, step-by-steps, and/or examples. The art director enforces the style guide and maintains the look.
Disney was a pioneer in this so they would be a good place to start. For games, the EA and Ubisoft stuff I worked on had varying quality to their guides.
The best style guide I’ve ever seen was for a game I worked on that got cancelled. It broke things down in such a clear way that I had an almost epiphany when I saw it come together. I can do my environment art prototyping about three times faster (not just on that game) after working on it.
Thanks a bunch guys! Really appreciate the help/insight on this
And for a more lazy answer, you could divide the work into environment and character art, if you have 2 artists and one does characters and 1 does environments, for if one does character portraits and cutscenes while the other does sprites you can tackle that obstacle by simply not having to make the comparison of the 2 styles for specific instances of a type of art asset.