First, it's the Behringer UFO202. $30 from Amazon. Cheap, right?

Now, remove the screws from the rear, and take off the top cover, revealing the insides:

We're going to be focusing on the area in the red rectangle.
These two traces between C18 and C21 carry the left and right signals from the input to the output.

The modification consists of cutting these two traces.

That's it! Put it back together, and now you've got discrete inputs and outputs.
I tested it with both Serato Vinyl and CD. For Vinyl, I used a 1200 Mk2 with an Orofon Night Club stylus. Obviously, the Line/Phono switch needs to be in the phono position. Depending on your needle, you may need to use a few ticks of the software turntable preamp. For CD, I used a CDJ800. I ran into an odd quirk here. The outputs of the CDJ800 might be particularly hot, or the mod might somehow make the line in super sensitive, but something was causing the stock Serato Control CD to overload the line in on the UFO202, preventing Mixxx from tracking. (Yes, the switch was in the line position.) In order to deal with this problem without having to whip out my soldering iron and some resistors, I simply attenuated the Serato Control WAV by 8.8 dB. That did the trick.
The line outputs on the UFO202 are a bit softer than you might be used to with pro DJ equipment, but you should be able to work around that with input trims. So, there you go! $60 for a Vinyl/CD control interface! Hope this helps you out!
