Coz its in a loop, so to speak, thats like saying, if you have 2 ABS sensors and 1 stops working, why dose the TRC light flash and not work, you still have the other 1 so should still work.
So to sum up, you get both lights telling you, you have problem, and in this case, coil pack gone, so in turn is affecting you TRC, so that lights as well :D