Both are absolutely hens. At 24 weeks, there would be a night and day difference between genders, not just a comb size difference.
Chickens are just as much individuals as any other animal. Some individuals have larger combs, some have smaller, some have a spot on one foot, some have bigger earlobes than the next hen, etc. Its very possible that your second hen will always have a more impressive comb than the first. It is also just as likely that she is growing a bit faster than her sister, and therefore her comb is showing her laying condition.