The Gemara discusses 2 men, 1 tahor & 1 tamei, who walked down two paths, one of which had a mes (corpse) and one which didn't and we don't know who went on which. This is compared to the case of a woman who loaned her garment to another and it came back wih a kesem (stain). There is a machlokes between Rebbi and R. Shimon ben Gamliel & one says that the owner can be toleh that the spot came from her friend who is shomeret yom kneged yom. There is the same machlokes here between Rebbi and R. Shimon ben Gamliel whether both men who travelled are tameim or just the already tomei one remains tomei.
How can we compare the 2 cases? Kesomim are d'rabbanan and tumat mais is d'Oraysa. We know that sofek d'Rabanan is l'kula but sofek d'Oraysa is l'chumra. Why would we say that the man who was tahor is still tahor? It's a 50/50 chance that he took the road with the corpse on it and is now tmei meis.
Excellent point. In fact, we only conjecture that "the Tamei became Tamei" in a question that involves a point that is only mid'Rabanan.
In the case of the two paths, thought, the case is not actually a question of Tum'ah mid'Oraisa, as Tosfos (DH Banu) points out, since it is Safek Tum'ah b'Reshus ha'Rabim, which is Tahor mid'Oraisa (but is Tamei mid'Rabanan in this case, see Tosfos). If so, it is indeed comparable to Kesamim.
Be well, and Kesiva va'Chasima tova to you and yours!