Came back to say:
You don't *have* to have a confrontation. Sometimes that just creates drama.
(I'll admit, I was annoyed at them when I posted my first response, so that colors the aggression a bit. Sorta obvious, right?)
Especially at the front desk. Just be vague. Don't stop.
Walk in, wave, KEEP WALKING. (Remember, you are the one w/ feet; she is stuck at the desk and can't follow you to talk.)
Say "Hi, how are you, gotta go!" And keep walking.
And then, in all the other encounters, you and DH can use the same tactics.
He goes over to their house to drop something off; he's got feet, he can say, "can't talk right now, I need to get going," and walk to his car and *drive off*!
One of them comes to your house, he puts his hand on their arm in a friendly way and says, "I've got stuff to do, and I need to concentrate, so I'm going to send you home now."
They can go on and on about whatever has ticked them off, but you never, ever, ever address it. Regard it as, oh, a dramatic monologue which would be rude for you to interrupt.
And remember this: every time they vent to you, and you sympathize, you reinforce their quarrel. You reward the bad blood, the griping, etc. They feel good when you listen to them and sympathize with them. You aren't actually helping them.
You may think you're being nice, and being kind, etc. But it's like with a toddler. They want something bad for them, and they're upset when they don't get it. If you sympathize with the upset-ness, all you do is reinforce the idea that they *should* be upset that they can't play with the kitchen knife.
You also send the message that you think they're *right* in some way. And you take away the opportunity for them to actually look at their demand, or their fight, and second-guess themselves.
So another possible response to them, which might be easier to maintain, is to be dismissive:
"You guys fight all the time, and you always get over it. This is nothing, I'm sure it'll be gone tomorrow."
That might even be a great "cut-and-paste" line with them.
It might take practice, but that's the road to peace.