TL;DR: Multiple Sunrise Technique traits don’t stack, despite the tooltip saying so; instead, only the highest-ilevel item’s Sunrise Technique applies damage.
While manually simulating attacks I noticed that if you have Sunrise Technique on multiple items, the tooltip combines the two items’ numbers but in practice it only procs on the higher-ilevel item. Meaning if you had an ilvl 370 ST item (procs for ~400) and an ilvl 400 ST item (procs for ~600), it’d damage your enemy only for 600 rather than 1000. The tooltip will claim to proc for 1000, but the combat log will show it doing only 600 damage. (Which was exactly my case)
Looks like SimulationCraft follows the expected (but currently bugged) stacking behavior so keep that in mind when you sim gear.