I've now spawned duos of treeguards two times, both in the same world. Both times were during full moon, in adventure mode (a cold reception).
At first I just though spawning doubles is like cracking an egg and finding double yolks. Generally, I've been spawning a hecka lot of these guys, so lots of chances, but still, I find it a bit unusual. Anyone else been spawning doubles during full moon or cold reception?