The Nero/Castellari project has been in the talking stages for many years.
If you ask Enzo if the movie is still in the works, he will say YES. Even though they haven't even begun any real work on it. I asked him about this just a few months ago and he replied the same way he replied when I asked him about it in 2004: "we are just working on the budget".
So, who knows when that project will ever get off the ground.
Also, in 2004, Aldo Sanbrell showed me a photo of himself with Gemma, Terence Hill, Tonino Valerii, and Bud Spencer (who was almost cropped out of the entire photo for some reason). He told me at that time they would be making a Western in Almeria the next year. But, it didn't happen........
So, maybe it is the same with the Spencer/Gemma film. Maybe it was something that was planned and they might have even got around to shooting a few things; but, it might be on hold for one reason or another, at this point.
The movie business is a great deal more than fickle, let me tell you!
I, myself, have been "on the line" as the composer for the film HIRED GUNS (which was, at one point, to star Franco Nero) since late 2005. The project has gone through four or five different incarnations with just as many production houses and cast changes!