Listen, I don't claim to know your life, but if you've ever needed to record audio with two microphones that are fed directly into your DSLR camera, then here's a really great solution for you. Caleb Pike of DSLR Video Shooter has come up with a super simple and ridiculously cheap way to do just that by using a $5 splitter cable, which will not only allow you to use two mics simultaneously but will also keep the two channels separated so you can control the levels independently in post. Check out the video below:
Now, before you go out and buy up just any old splitter, know that most of them come with left and right cables for both ends (amounting to 4 cables total on the end). The one Pike uses in his video only has a left and right cable, so you can easily connect two microphones. Keep in mind, though, that this has a 3.5mm plug, so you won't be able to use it with XLRs without an adapter. (You can find the cable here.)
This is really an awesome setup, because the cable allows you to record two different microphones to two different channels. So, say you wanted to use a lav mic and a boom mic—this cable will let you record not only from both at the same time, but record two separate channels that you can manage levels and add effects to in post.
It's a really simple and cheap solution to a pesky issue many of you might be having. Plus, you won't have to deal with a preamp or external recorder, so—you know—that might be an added bonus.
Source: DSLR Video Shooter