I only have one chin, but I placed the litter pan in the spot where she was peeing. I placed litter in the pan, as well as soiled bedding. I blocked other corners of the cage to deter her from going there. I am not an expert, but you may have to start with two litter pans and place them where each chin goes potty, even if for now it is in the doorway. When they both get used to the idea of the pan, you could try removing the one in front of the door and leaving just one litter pan for the two to use.
Good luck. I know some chins won't use the litter pans, but I got lucky and my chin got the idea quickly.