I currently shoot using a FujiFilm X-T1 and have a 35mm 1.4 , Zeiss 12m 2.8 and cheap 50-230mm slow zoom.

I largely shoot animals, generally dogs and often dogs in action and am finding the X-T1 a bit frustration at focus tracking and in low light acquisition.
I was keen to get the 50-140mm 2.8 Fuji lens but will likely be stuck with the same frustration.

This lens is around $1800 + so it's a fair investment into the system.

While the D750 looks like a great value option it's a bit price restrictive, that and I like the added reach a crop body gives. Though reduced subject isolation and especially in the case of the D750 low light performance is a bit of a downer.

So I think the D7200 isn't a bad option? Coupled with the Tamron, which buy all the reading of done is well regarded and great value, even some about for $700 second hand, the combo of a new D7200 (can't find any second hand) and Tamron 2.8 may come in only marginally dearer than the Fuji 50-140mm 2.8 on it's own, new (again non second hand).

The D7200 lacks a few things which would annoy me as a sole body, ie lack of tilt screen which I find massively useful on the X-T1, but using it largely with just the 70-200 view finder only would be fine.

From what I've read it's image quality and sensor are pretty much top of the APC options, the X-T1 is also highly regarded and back to back comparisons quite close, hence why I've considered the Nikon over Canon options, which seem to be a step back from Fuji or Nikon image quality.

I've already read that while on paper the Nikon focus 'specs' aren't quite a 7Dii , in actual use people find their 3D/Subject tracking a focus system better? Is this true in anyone experience?

I'd be keen to hear anyone using the D7200 and even with that lens combo for similar type purposes and if the idea of having two bodies of different brands somewhat for different types of shooting is a good one, or if I should maybe try sell the Fujifilm stuff and move too a single system?