This should fit without any problems. I have an LG 34GN850P, which at 819.20 mm is just as wide as the Odyssey G8, and a 32" monitor connected to the arm - although the latter is upright. But there is still plenty of room for manoeuvre and I think you could actually connect two 34" monitors if you wanted to.

I have a Dell C3422WE (816mm wide): it doesn't extend beyond the end of its arm; with two of them I would probably have just under a hand's breadth of "reserve". The Odyssey G8 is a bit smaller and you have enough room to arrange the two monitors nicely :-). I can only recommend the stand - have fun with it!

I have the Alienware ultrawide and the odyssey g6 side by side. I doubt that two ultrawide monitors will fit horizontally next to each other. I advise you to buy 2 single arms, I had that with another office desk and it worked much better.