My only experience thus far is with the 5D3. However, from reading all the forums, your paid gig shooting dogs would benefit from the 1DX2 since it sounds like you had trouble with keeping up with the 5D4 (from the FM forums). While the 1DX2 may be a little noisy for weddings, I can imagine it can't be that bad. But not sure. I say keep the 1DX2 since the 5D4 is not fast enough for your dog work. And it should be fine for your other paid gigs and personal stuff.
Dual 5D4 cameras, or one 1DX2 and one 5D4, would be nice but expensive. And you have to think about what benefits you would have with a second body. Would you really carry around two bodies frequently to cover different FL ranges? Is the second body just a backup?
If all you're looking for is a backup, the 5D4 is an expensive one to just have it sit there in case your first camera breaks.
Again, this is only my experience form reading other people's experiences online lol