(a) The Mishnah rules that if a Sa'ah of Terumah fell into one of two boxes or storehouses each containing fifty Sa'in, but it is not know which one - the two combine to be Mevatel the Sa'ah ...
(b) ... leaving the owner with the options - of taking out a Sa'ah from either storehouse or box, or half a Sa'ah from one, and half a Sa'ah from the other.
(c) According to the Tana Kama, this ruling apply even if the boxes are in two different rooms - as long as the boxes are movable, because since it sometimes happens that they both end up in the same room, we consider them as if they were (see Tiferes Yisrael).
(d) Rebbi Shimon says - that even if they are not movable (and presumably, the Heter extends to storehouses too.
(e) The Halachah is - like the Tana Kama.
(a) Rebbi Yossi cited an episode that occurred with Rebbi Akiva, where one bundle, comprising half Terumah and half Chulin, fell into fifty bundles of Chulin more or less the same size. The total amount of Sa'in in the box was - a hundred and two halves (including the half Sa'ah of Terumah that fell in [see Tosfos Yom-Tov]).
(b) Rebbi Yossi suggested to his Rebbe - that the bundle that fell in should be Batel ...
(c) ... despite the fact that half the bundle was Terumah (and we might have thought that the entire bundle is considered Terumah, in which case there was not a hundred to be Mevatel it).
(d) By virtue of Rebbi Akiva's silence, it is clear - that he agreed with his Talmid's ruling.