We have 2 blue tit hens sharing and incubating 18 eggs in a nestbox with a camera. Does this happen very often

Last week we noticed 12 eggs were laid in one day and then a further 3 the following day and now we have 18 eggs. We have also noticed that 2 hens are simultaneoulsy incubating the eggs on the nest together. They seem to get on reasonably well together but sometimes one bird sits on top of the other. Occasionally a male comes into the nest to feed a female.

Has this ever happened before and what is the likely outcome? I can't imagine it would be a favourable outcome if all the eggs hatch.