Taeui is like a good luck charm or gilsangcheon to His big brother twin Jaeui,,so whenever,Jaeui gets in danger or kidnapped,,he would be safe or will back safely,,if taeui is safe too,,but if Tae is in danger or hurt,, Jaeui will also be in danger,,if taeil die,,jaeui will also,,just like they described how Jae used to fell ill whenever Tae used fell ill ,, also as you saw how The gun did not work when ilay tried to shoot jae but backfired at him,,it mainly because of Jaeui's luck,,,but there is no mention that whenever tae get injured,,jae will feel the pain..but it mainly affects jae if tae is in life threatening danger or death,,,
Hope this helps,,i also recommend reading the novel,,because this explanation is better in there
So everytime one of them feels pain, the other feels it too? Tbh I dont really understand all abt the gilsangcheon thing