"Someone's a real witch today." Bill says. ""Man what's her deal?"
"I know," Lavell commiserates. "And if you say anything about it, she'll bite your head off."
"Maybe she got in a fight with her boyfriend," Bill remarks.
"Yeah. Or it's that time of the month..." Lavell trails off and the two of them go back to work.
Discussion
The manager needs to talk to these employees individually. Say what was overheard and let them know it's inappropriate because they're making assumptions.
Ask, "What was your purpose in saying that? What good will comments like that do for our team? For our organization?"
All this kind of thing can do is hurt. Tell them to stop the behavior. Then set a challenge: If they want to productively fix behaviors, they should talk directly to the person.
Instead of talking behind another's back, they can state honestly, "We seem to be butting heads here. I'd like to change the situation and I want to know if you have ideas what causes this tension."
Once it's established that they need to stop the negative behavior, give employees the freedom and the tools to deal with each other.