We normally see that, other than in laying hens, broiler breeders have more difficulties using the upper layer of a two tier laying nests. Its is not equal for each breed, body weight, condition, leg strength and training in rearing plays a role as well, but in general the top layer of a two tier laying nests is only used for 50% of the actual capacity. Birds use the lower layer more, especially when the perches in front of the nests are too narrow or when the distance to jump to the second layer is too high.
This means that when we calculate the available space, we cannot calculate the upper layer as a fully available layer of nests. When we have a set of 12 individual nests divided in 2 layers of 6 nests each place on top of eachother, a more adequate calculation is probably to consider the lower level to be fully available and the upper layer for 50% available. This means that a set of 12 nests should be calculated in this situation as 9 nest boxes, 6 from the lower level and 3 from the higher level. When we place 6 birds per nest box, this will result for a section of 2 x 6 nests not in 12 x 6 birds = 72 birds per section, but 9 x 6 birds is 54 birds per section. If we calculate the availability as 100%, we might create more floor eggs due to overcrowding of the lower level of the nests.
Again, it will depend a lot on the actual situation and the condition of the birds, but when there are floor egg problems with a two tier laying nest, this is a factor to consider.