My solution is to use a Tascam DR-100 recorder to capture wild sound. This is a discontinued box (Tascam has newer units that do the same thing) which provides dual XLR inputs and 48V phantom power that these mics require. XLR inputs are SO much better than miniplugs or phono jacks: they're reliable, stay put, and never hum. I plug one mic into an XLR cable, and then the left channel, the other mic into the right. Then record the sound into a 48K 24-bit wave file, and sync it up in editing. With a little practice, it only takes a few seconds with the editor (I use iMovie, but any decent video editor should do). Using the built in sound track as a reference, you don't need to "clap" or do anything silly that might upset your subjects. Just adjust the timing until the wild audio and the built in audio sound like one person, without a perceptible delay or echo (then turn off the built in audio). Just remember to start recording audio before capturing the video.
The sound improvement over the built-in mics is substantial: first the lavs are much closer to the people speaking. You also get much better stereo separation. Finally, you can set the levels and not be slave to the built in AGC in the camera. That's why even decent microphones that plug directly into a DSLR often sound really noisy. It's not that mic at all, it's the AGC going crazy during silent patches.
As someone else mentioned this kit contains an FR-350 (omni) and a FR-351 (cardioid). In the sample you can play, the left channel is the 350 and the right channel the 351. I think the 351 sounds a little better, but both are completely adequate for what I need. To get two decent XLR lavs at this price is a real bargain. (Sorry about the quality of the video.. it was originally HD, but wouldn't make Amazon's filesize cut.)