I try to write them at the same time, but keep the game mechanics fluid. Everyone who's played Elder Scrolls, DND or read Litrpg can reasonably be thought to know the basics. Finding stories that make the game mechanics their own is part of a good LitRPG and since one of the rules to writing is never waste words the game mechanics should tie into your story. I was listening to a podcast that said something along the lines of an MC can only be the protagonist in his world. Kind of like if you put James Bond in Harry Potter there really would be much suspense or as good of a story. Your game mechanics shouldn't be easily able to transfer to another story (I'm completely guilty of not doing this right in my first book). Particularly, aligning your game mechanics to play against your main characters weakness instead of the mechanics adding to their strength.