Communication is so important. If you don't have a pitch bible/rulebook of your story's/character's structure of what it is, what it is going to be, and what it will never be- make one now. It doesn't matter who you think you'll bring to the project, be it friend or trustworthy stranger- create your pitch bible to have details that someone ignorant to even the genre can understand. You want to eliminate any possible future confusion or conflicts.
If you have never done something like this before it may feel weird and even selfish- but doing this is not only for you, this will be appreciated by any serious collaborator. Do not worry about embarrassing or "overstepping" rules when establishing what you are willing to work with. Any honest fellow creator will understand you are looking for the right fit, anyone who tries to make trouble you know you didn't want in this project anyways.
Story elements you are flexible with and don't mind tweaking if your partner has a better idea- you can make list for that too (You'll likely get better info from your interviews in general though). If you cannot pay your partner in money, make sure you can pay them in some value of a story/character idea that they can latch onto for however long this partnership lasts. Establish possible endings so neither of you get caught off guard about accidental favoritism of side characters/themes.
Also no matter how casual, friendly, etc the person is, please think about a contract. Yes, even if the project is not set for money. Both of you are going to be investing time into a large project. You will both be safer for it.
In some ways you are already ahead of the game because you have 2 years worth of proof that you are serious about this project! Best wishes in finding someone!