It sounds similar to rolling items into quarters, etc. for financials.
I did a quick search for the quarters, etc. in the community, but think that each are helpful (some qlikview, some similar topics).
I think it would be adding two measures: A) 1 being months 1 <= x <= 6, B) being months 7 <= x <= 12)
Community posts that may be helpful:
Consolidating Months into Quarters
Show data for week,month,quarter,year based on date entered by user.
Help Calculating Period-over-Period Growth