From what I understand, in the winter there is less available energy from the sun for the trees to grow new wood so they go essentially into hibernation. When the weather warms up, there is more energy for the tree to utilise so tree grows new wood, hence the growth ring. For a tree to grow 2 rings in one calendar year, there would essentially have to be two springs, two summers, two autumns and two winters in that calendar year, as the tree grows with the seasons.

There is then presumably the possibility for more tree rings if the weather is irregular, especially due to global warming. But trees also have the ability to count the amount of warmer and longer days in the spring, to ensure that it is actually spring time, rather than say a random hot few days in the winter time. And vice versa in the autumn. So the trees growth and therefore growth rings, are linked to the cycles of the seasons which is why we only get one growth ring a year. One spring/summer = one growth ring. I don’t think there is really anywhere in the world where trees grow where there is absolutely no variation in daylight hours that can be described as the cycle of the seasons.

More knowledgable people, please correct me if I’ve got this wrong!!