I just bought a Gemini 2 and am running it in stereo. You might find it easier to use the XLR for the Gemini. In that case, you need a balanced cable that is 1/4" on one end and female XLR on the other. Then mate that with a Hosa YFX-119 (6") or YFX-101.5 (18") or equivalent Y cable to connect to the both XLR outputs on the AX8. That is how I'm running it with my Axe-FX III anyway. I'm sure there is some combination of cable and adapter to go from dual 1/4" TS to 1/4" TRS (delivering two mono signals, not a balanced signal). If you do it like this then get balanced 1/4" TRS cables to send to front of house. If you want, you get just get short ones and connect them to regular mic cables.
I agree that not having two (ideally balanced) inputs on the Gemini 2 is lame. I'm sure there are some technical constraints but whatever, work it out. Also, Mission makes it less than easy to understand how to use the stereo option. It took me a while to figure it out and even longer to find the Y cable.
I have also used the USB to send a stereo signal from my computer to play program material, and I have to say it does sound pretty awesome. When your wife or girlfriend complains about how much your Gemini 2 cost, you can say, "but babe, we can blast tunes at our parties".