Theoretically, optically, it would work (for low-quality values of "work").
Yes, the focal reducer would give an extra stop of light. But because the 2x teleconverter is responsible for 2 stops of light loss, you'd still net 1 stop of light loss.
Aside from the optics math, there are some real practical problems with this approach. Depending on exactly which 50mm lens(es) you're talking about, some of them have rear elements that sit quite far back. And depending on which teleconverter(s) you're talking about (especially 2x teles), the teleconverter might intrude into the lens's space. So it is very possible that a 2x tele will not mate a certain 50mm lens.
Generally, wide-angle and normal focal length lenses are not meant to be mated with teleconverters. It's tempting to think of lenses, filters, and teleconverters as sort of snap-together, Lego-like modular components that can be stacked together and added onto. Physical mating-wise, that is true. But everything you add in the optical path has drawbacks. And items that weren't specifically designed to work together will have worse combined drawbacks than items that were meant to work together.
Teleconverters have real, visible downsides (loss of light, some degree of loss of quality or sharpness, etc.). Those downsides are acceptable to turn a 200mm lens into a 280mm or 400mm lens, without having to carry such a monstrous beast (with the corresponding monstrous price). But those downsides will quickly turn a nifty 50 into a crappy 70 or 100, when those are already readily available without compromise.
