Two Types of K’riat Shma at Once
It emerges from the gemara that one time zone can be utilized both for daytime and nighttime k’riat shma. This prompts the question (see Pitchei Sh’arim to Brakhot; and Piskei Teshuvah III, 347), if one person is reciting the nighttime k’riat shma, and the other the daytime one, can one fulfill his obligation through listening to the other (assuming shomea k’oneh works for k’riat shma.)
The Kog’aglover Rav (Siach HaSadeh p. 51) considers the possibility that this would be comparable to the ruling of the Radbaz (cited in Magen Avraham O.C. 108) that one who needs to make up ma’ariv can do so by listening to the chazzan’s repetition of shacharit, despite the fact that they are two different prayers. He rejects this possibility, though, noting that the ma’ariv-shacharis case, there is nothing inconsistent with both coexisting in the same time period; ma’ariv is meant to be made up during shacharit;s time. K’riat shma, though, has no such arrangement; and the recital of shma as a nighttime fulfillment is mutually exclusive (tartei d’satrei) with a daytime fulfillment.
Similarly, R. Avraham Weinfeld (Resp. Lev Avraham, 8) rejects a similar comparison suggested between this situation and that stated in Shulchan Arukh (O.C. 588:4) that one shofar blast can be used by different people to count for different stages within the obligation of shofar. There, observes R. Weinfeld, both aspects are part of one obligation; here, nighttime and daytime versions of kriat shma represents two separate obligations. This is true according to both the Rambam and the Ramban; their dispute was only regarding how many mitzvot out of the 613 kriat shma represents, one or two, but they both agree that they are two distinct obligations.