Flat-screen TVs have gotten cheap enough that it's possible to get two midsize TVs for the price of one bigger TV. A quick search of Amazon found that you could get two 46-inch plasmas for the price of one 52-inch LED LCD. Or, you could add a new, smaller TV to supplement the one you already have for only a few hundred dollars.
Why, you may ask? A better question: How (would you use them), and even more important, what do you need?
Let me first say, beyond a boring thought experiment, I actually do this. It is awesome. My setup isn't typical, nor possible for many people, but it does give an idea of why you'd want two TVs. My main display is a front projector (again, this is just an example, you can do the two-TV thing with two HDTVs just as easily). For the most part, I use the projector for everything: TV watching, movies, gaming. It's the latter that makes the a second TV useful. There's a lot of downtime in games like Star Wars: The Old Republic or Battlefield 3. I'm certainly never bored playing these games (otherwise, I'd do something else), but my brain loves multitasking. I hooked up an old 42-inch LCD and placed it to the side of the main screen. An Apple TV is all that's connected to it, so I can watch Netflix. For example, I recently watched all five seasons of the incredibly brilliant "Friday Night Lights" while leveling up my bounty hunter in SWTOR.
Another use could be sports. Two games at the same time, so you can monitor both without having to switch back and forth. Think a sports bar or Vegas, with a wall of TVs showing all the games at once. The "wall" in this case is just two TVs, but you get the idea.
This gets a bit tricky, and depends on what you want to do. I'll leave the TV picking to you, as that's the biggest cost. For me, a main, large TV and a smaller "satellite" TV makes the most sense. Personally, I think getting two TVs that are the same size makes less sense, but to each their own.
The sources are the hard part. In my example above, my main display (the projector) is connected to a receiver that switches all the sources. The 2nd television is connected directly to the Apple TV. If I want to watch the Apple TV on the big screen, I have to swap HDMI cables. Hardly ideal, but it is simple and cheap.
One of the easiest methods to get content on the secondary TV is a simple antenna. All modern HDTVs have a built in HDTV tuner. You should be able to get free HD with that. For more on this, check out my How to get free HDTV article. Another option is a Blu-ray player with two HDMI outputs.
The tricky part is audio. With gaming, it's not a big deal to have both audio streams going at once. In my case, the HTPC runs through my 5.1 system, and the Apple TV goes through the smaller TV's speakers. In the cases where I need to hear something important in the game, I can pause or mute the TV. This is where thinking of one as the "main" TV and another as the secondary TV makes things easier.
If you want two TVs of equal importance and still want decent sound, using the audio output of the TVs as the audio source to send to different inputs on a receiver is the simplest solution. Then you just need to switch inputs on the receiver to hear the TV you want. Note that Audio Return Channel (on HDMI) won't work here, as you're using a Matrix switcher for the HDMI.
With this method, the speakers will still work on the TV, so you can use them as an option as well.
The last consideration is controlling everything. If you buy two TVs of the same make, whatever button you press on one TV's remote is going to do the same on the other (as in, volume, mute, etc). For that reason alone, getting two different brands is probably a good option. However, it's not quite that simple. Some remotes work across brands. Vizio and LG remotes often control each other's televisions, as do some Samsung and Sony models. It's probably worth a trip to your local TV store to test the remotes of the TVs you're considering (presuming the store has the remotes available).
A decent programmable remote is probably vital unless you don't mind a litter of remotes (what is the term for a group of remotes? A pack? A herd? A murder?).
There are custom install options that do IR commands over wires, but at that point, I recommend talking to a professional installer.
Two TVs in one room is hardly for everyone, and I'm certainly not trying to convince anyone to try it. I enjoy watching TV and gaming at the same time, and if you think you'd enjoy the same, this article was for you. Have you already built a two-TV setup? How'd you do it?