פירוש על קידושין 101:4

Cattle tithe is different because an animal can be considered tithe even if one makes a mistake in counting. Just as in the mishnah cited here, animals that are not the tenth can be considered tithe, so too in Rava’s example of the tenth and eleventh coming out of the pen together, both can be considered tithes. But kiddushin cannot be done accidentally, and therefore, if the kiddushin cannot be done one after the other, they also cannot be done simultaneously.