Why does falling on an article not effect a kinyan ?, the gemaro says it shows that he does not want the kinyan of 4 amos to take effect.
In contrast we can say that if 2 people pick up an article the one has in mind to be kona for the other because otherwise he will be kona nothing. He certaintly does not want the other person to be kona he wants it for himself. So why when he falls on an article we do not say he wnts to be kona any way that is legal?
When two people lift a Metzi'ah together, each one wants his Kinyan Hagbahah to take effect in whatever manner it can take effect. Since the only when for one person's Kinyan to take effect is to be Makneh have of it to the other person, this is included in his intention.
In the case of the person who fell upon the object, it is his desire for a "Kinyan Nefilah" to take place, and not for a Kinyan Arba Amos to take effect on the object at all. (Moreover, he has intention that the Kinyan Arba Amos not work, since it is only a Kinyan d'Rabanan, and he prefers a Kinyan d'Oraisa, as the Rishonim explain Rashi). Therefore, the second Kinyan does not take effect at all.
M. Kornfeld