Anything that cannot be accomplished sequentially cannot be accomplished simultaneously — Eruvin 50a
תלמוד בבלי מסכת עירובין דף נ/א
כל שאינו בזה אחר זה אפילו בבת אחת אינו
The Tashbetz (3:147, cited here in Gilyonei HaShas) writes that the reason why anything that cannot be accomplished sequentially cannot be accomplished simultaneously is because such a case is analogous to the case of: "You and the donkey acquire this" (Bava Basra 143a), in which the law is that just as the donkey cannot acquire the object in question, so too the person also does not acquire it. Here too, since each one of the transactions is precluded by the other, therefore, each one is, vis-à-vis the other transaction, is like a donkey, and therefore each prevents the other from taking effect.
This link is reflected in a case in the Mordechai (Bava Basra §605): "Once upon a time a person simultaneously transferred to his friend ownership over real estate, movable objects and coins all together, through a kinyan sudar [chalifin, the same type of kinyan we use to appoint a Rav as an agent for Mechiras Chametz — such a kinyan is not effective in transferring the ownership of coins]. The great rabbis of the generation disputed the law in this case. Some said it was like the case of "You and the donkey acquire this"… and that just as ownership over the coins was not transferred, neither was ownership over the real estate and movable objects. Others said that since the transaction was effective for the real estate and movable objects, it was also effective for the coins. Rabbeinu Simcha wrote that although the transaction was not effective vis-à-vis the coins, it was effective vis-à-vis the real estate and movable objects… and it cannot be said that this is like the case of "You and the donkey acquire this," as a donkey is not able to effect any transfer of ownership, while coins are subject to transfer of ownership by pulling (meshichah), lifting (hagbahah) and being placed in one's yard (chatzer).
The question may be asked, however, is not the halachah in the case of "You and the donkey acquire this" that the person acquires half of the objects or lands in question? If so, shouldn't everyone concede to Rabbeinu Simcha?
Bigdei Shesh (Bava Basra §46) addresses this issue, and explains that although these sources are borrowing the terminology from the case of "You and the donkey acquire this," the cases are dissimilar in an important way — implied by Rabbeinu Simcha himself. In the literal case of "You and the donkey acquire this," the transaction with the donkey is not a contradiction to the transaction with the person — rather, the transaction with the donkey is void and non-existent, and therefore does not impact on the linked transaction which is void and existent. In these other cases, however, the each of the combined transactions conflict with and contradict the other (indicated by their not being subject to being accomplished sequentially). It is therefore logical to conclude that in this figurative "You and the donkey acquire this," the transactions cancel each other and are both void.