Great question. If you're only going to do this once, then make sure you record the audio on the camera--it would be a hassle to sync audio recorded from another source later. But you don't have to use the camera's crappy microphone to do this.
Before I dive into my 2 solutions, let me just say, you can rent all kinds of good gear out there on the Internets and through you local camera store. Because this is just a one-time thing, it may not be worth buying something new.
Fancy Wireless 2-mic $etup:
I use two wireless battery-powered lapel microphones. Each of them has its own battery-powered wireless receiver. I connect these to a JuicedLink mounted to my tripod, and mount my Canon 60D to the JuicedLink. The JuicedLink functions as my mixer/pre-amp. One microphone goes to L, and the other goes to R. My camera records in stereo, so now I have 2 independent audio tracks to mix in Final Cut Pro X. This means I can EQ and Compress each actor/interviewee independently.
Shotgun 2-mic Setup:
This setup is around $1,800, camera and tripod not included, but fear not, there are lots of ways to record 2 microphone inputs in stereo. One way to cut cost would be to drop wireless in favor of wired. Another would be to drop the JuicedLink.
For example, you could purchase 2 battery-powered shotgun mics. Shotgun mics have a highly selective hyper-cardioid pickup pattern. They also have a lot more range because they are phantom-powered. (That's also why you need batteries; don't count on getting that power from your camera.) They could mounted on mic stands and wired like this:
[Shotgun 1]--(XLR)--[XLR to 1/4" mono]--[1/4" Y]--\
[Shotgun 2]--(XLR)--[XLR to 1/4" mono]--[1/4" Y]--/
Bottom line: If you want to mix 2 channels of audio, record the audio on the camera. That way, it's already synced with your video. And if your camera can record in stereo, split it left and right.