as i only race old cocks im really hoping 2 cocks in the nest from 1 or 2 hens the other way around i have not kept such a close eye on but some of my best breeding hens have never thrown 2 hens to the best of my memory even when switched around with other cocks i rarely leave pairs together if you get the book by vansalen it goes into depth on chromosomes etc its very interesting i myself just try to put the best to the best and hope for a bit of luck if im right the hen and im most probably not id have to read up on it again the sex of youngsters is down to the hen is it down to the timing of the mating im not sure