Rent one. You're 1-3 days away from LensRentals.com as FedEx goes. But me personally, I rarely find that the next model is all that worthwhile of an upgrade, and it usually takes two revisions before I'm jumping to buy up. If anything, I'd be looking at a 5D4 or 5DsR if I were in your shoes, and the only reason I'd be looking at a 1Dx2 is if you needed the responsiveness that the 5DsR just barely lacks or the ISO that the 5D4 can't give you.

If it's any perspective, my wife and I both shoot. We have a 5DsR, two 1Dx, and a 5D3. Normally, I get the 5DsR and a 1Dx, as she's not good at managing her exposure and will often spend a day either at ISO 6400 or f/18 (or both), and the 5DsR doesn't do well at max ISO or at long shutter speeds, so I just don't let her use it. She therefore gets the 5D3 and a 1Dx. However, I get twitchy whenever I think about some random failure in the 5DsR, so I ponder the right choice for a backup camera. The 5D4 seems to fit the bill perfectly: 30mp feels like a better 50mp backup than 22 or 18mp, dual CPU seems like a better backup than single (in the 5D3), and yet "reasonable" high ISO performance makes it "safe" for my wife to use. I do suspect there's another 5DsR in our future after the 5D4, but I'm just not seeing myself reach for the 1Dx often enough to create a case to get a 1Dx2. Does that perspective help at all?