I'm a landlord and my tenant emailed me that in her bathroom, each of the two sinks has the following problem: One time, the left knob is hot water and the right is cold. Later, the left knob is cold water and the right is hot. This problem has been going on for "a while now," and it has been increasing in frequency. And it is only happening in the two master bathroom sinks.
I've been unable to find any mention of this problem, let alone a solution, by Googling. Does anybody have an idea what's going on and how to fix it? I could just call a plumber, but I'd like to fix it myself if possible.