Can I split audio before it gets to my sound recorder? In the past I have been using a wireless lav for interviews recorded directly to my DSLR (Canon 60D).  I recently switched to a double audio system using my Zoom H4. 

One  suggested setup was to channel the wireless lav directly to the Zoom then to split the headphone audio out of the Zoom to both the headphones and the DSLR audio for reference. However, this means the DSLR audio level changes if the headphone volume is adjusted. 

If instead I split the audio coming out of the wireless lav to both the Zoom and DSLR audio (Rather than splitting the output of the headphone jack), then the audio levels could be adjusted appropriately and wouldn't change.  Will that degrade the audio quality? Is that recommended?
Technically if you want to be sure that you are getting the highest possible audio quality you need a pre-amp that you can use to split the signal. But the effect is going to be very small.

When you split a signal you are relying on the impedance of the two input devices being high, and the output impedance of the source being low, and everything being even across the frequency spectrum. Most modern equipment has high input impedance and low output impedance, so that's generally fine–you will get a couple of dB drop because you're splitting the signal across two outputs–but what could go wrong is that you will get e.q. shifts because the impedances might not be perfectly flat across the whole range of frequencies.

However it is unlikely to cause much of a problem for recording interviews, most likely it would only be a real issue if you were recording something like musical instruments. In an interview things like the room acoustics are going to have a far greater effect on the audio quality. Either way, a splitter cable is only going to cost a few bucks, so it's worth a test.