Keritot 17A

והא דתניא חייב ⁠– בשגגת שבת וזדון מלאכות (דשבתות כגופין דמיין), דקסבר: אין ידיעה לחצי שיעור.

And that which is taught in the second baraita, that Rabban Gamliel deems a person liable if he wrote two letters over two Shabbatot, can also be explained: It is referring to a case where he acts unwittingly in that he does not know it is Shabbat, and intentionally in that he knows his actions constitute prohibited labors. In such a situation he deems an individual liable despite the fact that different Shabbatot are comparable to separate entities, as he holds that although the intervening days are generally considered awareness of his transgression, there is no awareness for a half-measure. Since he performed only half the prohibited labor on the first Shabbat, he is not considered to have become aware of his transgression on the intervening days.

אלא לרבא, דאמר: סבירא ליה לרבי עקיבא דשבתות כגוף אחד הן,

But according to Rava, who says that R' Akiva, and presumably also Rabban Gamliel, holds that separate Shabbatot are considered as one single entity, how can both baraitot be explained?

בשלמא הא דתניא חייב, משכחת לה בין בזדון שבת ושגגת מלאכות, דשבתות כגוף אחד הן, ובין בשגגת שבת וזדון מלאכות, דקסבר: אין ידיעה לחצי שיעור.

The Talmud elaborates: Granted, that which is taught in the second baraita, that Rabban Gamliel deems one who writes a letter on each of two Shabbatot to be liable, is understandable. You can find both a case where one acted intentionally in that he knew it was Shabbat and unwittingly in that he did not know his actions constituted prohibited labors, as he holds that Shabbatot are considered as one single entity, and a case where he acted unwittingly in that he does not know it is Shabbat and intentionally in that he knows his actions constitute prohibited labors. The reason in the second case is that he holds there is no awareness for a half-measure, and therefore the actions on the two Shabbatot are considered to have taken place during one lapse of awareness.