Gunsmithing KAC SR-15 E3 Trigger Spring Question

[IronMaidenFan]

As I understand it, my SR-15 ships with a 4.5 lb trigger weight. I wanted to lighten that up a bit and found 3.5 lb springs here.

I see that my trigger group has 2 springs. My question is do I need to replace both springs with this new "red" spring or just one of the original springs. If the answer is the latter, which of the original springs gets replaced.

Thanks!