Y'know what? Players are dicks. Well, maybe not your players. Hell, maybe not my players either. But that is how I wanted to start this post off. Why? Because sometimes, no matter how hard you try, they're going to try and listen in on a conversation between two NPCs. What does that mean? Well, it means that you get to find a way to RP two NPCs at the same time, in conversation. This is something I've never been good at, so I figured I'd go over a couple of ways that it can be handled. Maybe you'll have more, and better, ways to do it? (I hope so!)
Just Do It!
This is the most basic answer, and something I've seen numerous GMs (myself included) do to various degrees of success. In this way you get to just bite down on the bullet and do it. Two NPCs, one conversation, and go. Be prepared for a lot of brain switching as you jump from character to character. You're going to feel a bit like a crazy person (then again, what sane person GMs?) and if you normally do accents it may be worth dropping them for your own peace of mind. As much good they'll do helping keep each character separate, it will also cause massive confusion if you use the wrong accent for a small part of the conversation.
I would also recommend keeping the conversation brief and to the point. This is for your sake (you are doing both sides of a conversation after all) and for your player's sake. After all, while you are doing this, all eyes are going to be on you and no one is going to be able to do anything else. Not to mention that people - even when interested - tend to tone out long lectures from one person, even if it is silly. So, unless you're a professional entertainer, a long conversation is more likely to put your PCs to sleep then get them ramped up.
Write It Out
This is a faster way to do things because when the player involved is listening in, you just hand them a sheet of paper with the conversation already written out on it. Make sure to mark down tone indicators (calm, angry, sad, defensive) and such like that to help with understanding. If multiple people are going to be listening in, then make multiple copies. This is one of the places where tablets at the table can really help, because then you can just send the file on over. You can also give a complete conversation that people can read at their own leisure.
The problems with this method are two fold. One, it can only be done for conversations that you know are going to happen, and know are going to be over-heard and not interrupted. Two, you will likely have to take the papers back unless the PCs have eidetic memory or some way of recording it. Now, two may depend on your group, but generally you don't want players having the conversation verbatim to go over later. It stops all the fun of player memory and going off in hillarious wrong directions.
Assign The Roles
A variation of the above, though it can be done without a script, is to give the NPC roles to players at the table. Give each player a small briefing on the character they're playing, and have them act out the conversation. This solves the issues of players zoning out, because two people acting is a lot more entertaining than one. It can also keeps your players involved, as they're playing out this conversation. Extra bonus points as it lets the players take a trip into the heads of the NPCs for a little bit and see what is there, while giving you an alternate take on those characters.
The problems come in like before though. You need to be prepared for these characters' roles to be given out. Also, if vital information needs to be given out, you either need to specifically tell one of the players to bring it up, or you risk the chance of it not coming across properly.
You can summarize the conversation and give the players the highlights of the conversation. this gets the information across quickly, and saves you from having to act like a crazy person while RPing two or more people. On the down side though, you can also lose a chance to characterize the two NPCs that are doing the conversing, and it will be a lot less subtle. Still, it does work in a pinch.
That's All I Got
That's all I got on this. Do you have another way of doing it that works for you? For the most part I try to avoid having NPCs need to talk to each other on screen, but sometimes it does happen. It is just something that will have to be dealt with I suppose.