Speaking of alt costumes, did you ever notice that in RE1 in the costume room there are 2 different triggers to change costume yet only 1 alt costume per character? Prototypes show that they wanted to make 2 costumes per character but never got around to finish it, webm related

This is the reason why every port of RE1 adds their own second alt costume: everything for it was already programmed into the game, there is the second trigger in the costume room, and both Jill and Chris have another 3D model for that in the files except that the texture for them is identical to the original model. All the ports devs had to do was change that texture and that's it. Easy peasy.