The number of the glyphs ought to be even, though, otherwise there would not be the proper balance between the two halves, cfr 2 * 21 in Large Santiago and 2 * 10 in Keiti.

With an even number of glyphs the middle must be seen as two glyphs. The calendar in Small Santiago (for the year) has two identical glyphs in positions nos. 50 and 51. (But half of 100 is 50.)

The week in Large Santiago can be folded into two with 21 in the first half and 21 in the second. The middle must be the two glyphs 21 and 22 (although half of 42 is 21).

From this I cannot but think about the South American old way of having two kings. Perhaps also the custom in Middle America: "They went into fasting two by two, taking turns at carrying the tribes and all the Quiché people on their shoulders."