Rabbi Meir rules that the entire text of Torah, Nevi’im and Ketuvim can be written in a single scroll. Rabbi Yehuda rules that each of them must be written in a separate scroll. The Ḥakhamim rule that each of the books of the prophets must be written in a separate scroll by itself.
While Rabbeinu Hananel explains that the reason Rabbi Yehuda insists on separate scrolls is so that the holier Torah should not end up being under the books of navi or ketuvim, Tosafot argue that this would not explain the reason for separate scrolls for nevi’im and ketuvim. Therefore Tosafot suggest that the reason is simply to distinguish between them.
Rebbi (Rabbi Yehuda HaNasi) relates that a scroll with Torah, Nevi’im and Ketuvim was presented to him and he approved it. In fact, the halakha follows Rebbi, and we allow these disparate texts to be bound up in a scroll together, although they will not have the holiness of a sefer Torah, rather they are treated as ordinary ḥumashim (see the Rambam in Hilkhot Torah 7:15). These types of scrolls are not made with two handles like a sefer Torah, but with a single handle or pole. In order to protect the parchment and the writing on such a scroll, the baraita requires that a sufficient amount of empty parchment be left on the inside, so that it can fit around the wooden handle (ke-day lagol amud), and a larger amount of empty parchment must be left at the end of the scroll so that it can fit around the entire width of the scroll (ke-day lagol hekef).