Yes, thats right, that's the fact why I put both HD500-mixer-pans from "100%left" & "100%right" to "middle". That's the way to get a mono-signal out of both XLR.
Some mixers have not only mono-channels. They have some stereo-channels. But mostly not with two XLR. They use stereo 1/4" jacks for symmetric input.
So if your snake have enough XLR cores that you can use both XLR from the HD500, you need two adapters from XLR to 1/4"-stereo to use the stereo-channel of the mixer. :)
I play two instruments and use the dual-amp-function of the HD500. I use the left XLR for violin and the right XLR for guitar.
Some of my violin-sounds are not dual-amp, so I only use the left channel. Then both HD500-mixer-pans are 100% left.
Some of my guitar-sounds are not dual-amp, so I only use the right channel. Then both HD500-mixer-pans are 100% right.
When I practice at home with headphones I use a trick to hear both instruments in the middle of my head:
The last "effect" in the chain is the "fx-loop". If I play in church it is turned off. If I practice at home it is turned on and a short mono-1/4"-cable goes from "send" to "return R/mono"... Thats the way to get a real-mono-signal out of the HD500 - no matter which output. It's mono in headphones, 1/4" and XLR :D