I was testing the audio on the XL-2 today and noticed that if I connect an external mixer (Beachtek DXA-8 in my case for a GL-2) with a mini plug into the rear jack of the regular stereo mic jack I can get 2 channel audio via the single connection. It appears the rear jack has 2 channels of audio input.

Has anyone one tried this or am I the last to find this feature?

The Standard (stock) mic input has two holes (jacks) that corrrespond to the standard mic connector. The forward jack is a 2.5mm sub mini phone jack that provides 5 VDC (not phantom) power to the standard mic. The other connector is a 3.5mm stereo mini-phone jack that is the mic input and works with any mic-level source. This configuration was generally common the Canon line dating back to the L2, A1, Xl1, etc. although the pin spacing has changed a bit.