It is physically not possible for a bird's reproductive system to produce more than 1 ovum in a 24 to 26 hour time span.
Sometimes ( although fairly rare) two ovums will be released in quick succession, for whatever reasons. This usually results in a 'double yolker', if there is enough time in between the 2 ovum entering the shell pouch gland, then it would appear as though 2 eggs were layed in one day but it isn't physically possible for a hen to produce two ovums in a 24 to 26 hour period.
If it laid the first ovum in the shell pouch at 6 or 7am, then the second ovum at 5 or 6 pm, then it would appear as though 2 eggs where produced in one day.
When the first egg is laid, the follicle of the next ovum will rupture within a few minutes of that egg being laid, and thus taking 24 to 26 hours to go through the processes. Usually but not always, when the 2 egg a day myth arises the hen will skip the next 24 hours before producing another egg.
I had some melanistic pheasants that laid year round...not possible for seasonal breeders but it did happen, and sometimes I got 2 eggs a day, on occasion I got 3 eggs a day from this hen which I know isn't possible...'I thought'? So after researching this is what I found out about birds and egg production. Her reproductive system was really messed up. The second year of this hen laying this way resulted in 'shell less' eggs and finally EYP and eventually she stopped producing any eggs....laying for less than 2 years.
Still worth it I would imagine!
At first it seemed to work okay but then the medical thermometer seemed to become erratic. We decided it must not have been designed to reside in a hot, humid environment like the incubator for so long. (Oh yes... I did have water in the reservoir, to see whether the little button hygrometer functioned—it does, now that I’ve removed the plastic tab from the battery, but there seems to be no means to calibrate it.)
