Circles - 1/2 year age split

We created two classes in the 11-14 year age group - 11-12.5 and 12.5 to 14 - this worked to divide the age group into two harmonious and homogenous groups. The challenge is that we either can’t or don’t know how to reflect that division in the membership record and therefore the circles ... help?

I don't know as the membership system is designed to support a fractional age group. Keep in mind the progression happens on Jan 1 of the year the youth reaches that age. So, if your upper age group was 13-14, the child would advance on Jan 1 of the year they turn 13. Which would include all of your current 12 year-olds.

The Oct 5, 2019 First Presidency letter states:

Progression between Classes
As announced in December 2018, young women move from Primary to Young Women as a group at
the beginning of January in the year they turn 12. Likewise, young women progress between Young
Women classes as age-groups at the beginning of January in the year they turn the age of the next class.

So while I suppose a local Bishop/Stake President could determine that their specific situation is best served by splitting age groups as you've done, since the Church has stated that age-group-progression is intended, I don't think you'll necessarily find success in using the church resources to easily group the YW that way.