1. These two glyphs does show the fact that the year should be regarded as divided in two parts, each containing six months.
2. If we compare this with the similar glyph in Wednesday (as seen in Large Santiago Tablet)
where there are four wedge-shaped internal markings, I guess the common 'denominator' is the Moon. The Moon, I think, is the Giver of Time. Therefore with other time periods there is a need to 'translate' into the fundamental time period.
M / W = 4 and HY / M = 6.