It must be really difficult, though I guess in the majority of relationships one person will have the better job/the higher salary and I guess in a lot of relationships that dynamic can be very fluid, as here clearly she was in the ascendence when they first met, and it may be that she is again later. Fame is a fickle thing and it may be that Cox's fades again.
I guess with something like this you discover how strong a relationship is. It may be as well that what she's experiencing now might be what Brian experienced when they first got together and she was the 'famous' one.
I don't think that Cox's fame will fade in the forseeable future. Both David Attenborough and Patrick Moore stated that Cox is the natural successor for BBC's scientific programming and, as far as I can think of, there is noone else as suitable.
David Attenborough had his Life series and it looks like Cox might have an equally successful Wonders series.
That said, I didn't give any thought to how Cox himself might have felt when his wife was on TV and he wasn't.