Well I finally got them to spawn suddenly. It took emptying 4 dew collectors, and starting a forge and a campfire. Got back after a day in another biome. All 4 dew collectors were full. Emptied them, started up my campfire to boil water, started the forge to smelt, and BAM 2 screamers. I am guessing the dew collectors each stop generating heat when full, so all 4 generated heat immediately, then my campfire/forge started.
First time I have ever had two spawn at the same time. No torches or candles in the entire base.
Where do you see heat as it's generated? I didn't see it in the console view.