One of our cats keeps urinating in our bedroom, how can we tell which one?

We have two cats. One keeps urinating in our bedroom, including on clothes on the floor and on our bed. We haven't yet been able to catch which one it is. Is there any way to naturally dye one of the cat's urine to try and discover which cat it is? I have heard methylene blue will stain a human's urine, but will it work for cats in smaller doses? Also, if I order the fishtank methylene blue, which is a dropper, would it be safe to use for cats? Are there any other ways, like maybe just regular food coloring? Thanks!