In the very limited teaching literature available for four-mallet marimba technique, thirds and sixths scales are often ignored.
In some books (either intentionally or by mistake) the definition of thirds and sixths scales is also applied to double thirds and double sixths scales, which, obviously, require a completely different technical approach.
Thirds scales, in particular, should instead be studied, both with alternate sticks and with alternate hands, for two reasons:
because the alternate stick fingering (1-3 2-4) accustoms the player to using the four mallets in succession, even over minimum distances;
because alternate hand playing (1-2 3-4), as well as preparing the player for double thirds, improves the upward away movement, which is necessary in order to avoid damaging side shifts (see p. 19 and 20 figs. 7 and 9)