I don't really need a back-up cam but find one cam to shoot them all impractical. I do have a D3300 but it's hard to go back to DX when you're used to FX. I like my 14bit.
I'd like to add another FX as a slow-cam for my short lenses and use the D750 for fast shooting only. I like the D750 that much, a part of me wants another as a slow cam but I also realize that's an expensive way of not changing lenses. There's still three on my short list (D610 - D750 - D810) and any price changes in near future might influence which of them will be the winner.
It's a luxury problem.
Indeed, first-world stuff at its' finest... but count your blessings that we have this luxury.
Not sure exactly what you mean by "slow cam"... but if you are even considering a much more expensive D810, which is overall faster than a D750 (higher shutter speed and much superior sustained burst rate), then I would use the D81p as the main and the D750 as the slow.