Question about Canon EF 2x III teleconverter

I can't tell you 100% if this combination would work or not. But I would rather advise against it. Generally operate the Tamron 18 - 400 with a converter.
It will probably fit mechanically. On the lens side of the converter there is a lens element inside the lens. If the lens does not have the necessary recess, it will not fit mechanically.

In addition, I would like to point out. The 2x converter turns the 400mm into the desired 800mm. But even the smallest aperture of the lens (at 400mm without converter) 6.3 doubles. And then it can happen that the autofocus no longer works or works worse because it gets too little light.

PS: I would avoid giving pers. I would avoid giving out personal data like a customer number on a public site.

Certainly not fit. This teleconverter fits a few Canon L lenses (see text below). It also makes little sense to attach an expensive teleconverter to a low-light superzoom. Even if it would fit. You would get a 36-800 with f7-13 from the superzoom. 2X teleconverters are generally only recommended to a limited extent, because they also noticeably impair the image quality of top lenses (e.g. the EF 100-400 L IS II).

This extender extends the focal length of Canon telephoto or telephoto zoom lenses of the L-series by a factor of 2 with optimised AF performance and communication with a suitable camera-lens combination.
Not suitable for Canon EF 70-300 1.4-5.6 L IS USM and Canon EF 28-300 L F/3.5-5.6 IS USM