As I said in my first entry, some of my choices for "greatest crossovers" are not so much because the actual stories are great, but because the stories I choose say something to me about the special appeal of the crossover. Whereas crossovers in prose or comic books often have some "universe-building" elements, often those in feature films are much more haphazard, coming about when the producers of a crossover film are attempting to offer their viewers more for their money.
The powerful but short-lived subgenre of the "spaghetti western" generated a small number of mythic protagonists. Of those quasi-heroes, Django had only two "authorized" films while "Sartana" had five, but film-makers in Italy and Spain frequently appropriated the names and attached them to westerns that had no connection with the established characters. According to Wikipedia five non-canonical films were made teaming Django with Sartana; I've now seen three of the five, and am satisfied to pronounce 1970's DJANGO DEFIES SARTANA as the best of the three.
This oater, directed by one Pasquale Squitieri, is by no means a great western, but compared to the others I've seen, at least DDS makes an effort to adhere a little more closely to the personas established in the canonical films. The plot follows a predictable path. Bounty hunter Django's brother is hanged because he's thought to have been involved in a bank robbery. Django hears that the roaming gambler Sartana may have been involved in the robbery, and so Django goes in search of anyone who can prove his brother's innocence. Early in the film the audience learns that Sartana had nothing to do with the theft, and eventually Django learns this as well, though not before, in the grand tradition of Marvel Comics, the two crossover characters have a good dust-up. Despite this they team up against the real robbers and bring them to violent justice.
The two lead actors, George Ardisson and Tony Kendall, have a nodding physical resemblance to the actors who originated the roles. Neither one is particularly impressive, though Ardisson presents a strong visual presence with his dandyish clothes. This is just middling spaghetti-stuff, lacking the more frenzied scenes from the best of the subgenre-- with one exception. Sartana, cornering the head bad guy in his hacienda, shoots a pair of mounted antlers off the wall behind the villain, so that the horns fall on his shoulders, and for some reason get stuck there almost up to the point where the villain is killed. I don't imagine this was for any deep mythopoeic reason; the filmmakers probably just did it because it provided a bit of unusual visual business.
One scene shows a trace of sociopolitical awareness. While Sartana is drinking in the local saloon, a bunch of rowdies come in and hassle the black piano-player. A flag of the Confederacy is on the wall behind the rowdies, just to make the political point explicit. Sartana doesn't come to the piano-player's defense, but the lead rowdy senses a challenge from the silent gambler, and makes the mistake of getting in Sartana's face. After Sartana effortlessly kills the bully, the camera zooms back to the grateful face of the pianist-- easily this mediocre film's best moment.