Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 7:15:
מותר לדבק תורה נביאים וכתובים בכרך אחד .
It is permissible to attach Torah, Prophets and Writings in one scroll.
Between the end of Devarim and the beginning of Yehoshua, one needs 4 lines of separation, similar to how each of book of the Chumash is separated from the next one.
If Ramba"m mentions this, I assume that someone, at that time, must have done this. Or was Ramba"m just mentioning this as allowable, yet no one ever did it? Is there any evidence that someone at some time did this?