How much would it be for the second bag I'm flying from lax to Cambodia?

Generally, you can bring one carry-on luggage and total of your luggage cannot beyond 20 kg if you hold an economic ticket. And rules vary for different airlines, so you may check the exact info with your airline. This page is for your reference: https://www.travelchinaguide.com/china-flights/booking-faqs/3.htm#33