http://pokemondb.net/glossary this is the glossary , which includes egg cycles. Basically, a cycle is 255 steps. By using Rattata from the glossary as an example, it has 4080 steps to hatch, which is 16 egg cycles.
Flame body reduces the amount of egg cycles, therefore steps, it takes before the egg hatches.
I'm not 100% sure on the exact mechanic for the O-Powers, so come tomorrow (my time) I'm going to dig some decently heavy research into the exact way it affects, but what I think it does is make each step count has more than one, so having a 2x O-Power would mean each step would equal 2, halving the time it takes, or another way of putting it, making the eggs hatch 2x faster.
if I am correct, it would stack by Flame Body reducing the amount of steps it takes, and the O-power making each step worth more.
I'm not sure exactly how much Flame Body decreases the amount of Steps, I'll edit that in a moment.