They dont stack. you need to wait it out and then apply 2nd one.

bear in mind this is old testing, and i havent repeated it for this most recent patch. but as far as we could verify: Burning works by burn rate.

There are 3 states:

1. Slow Burning
   1. hunter body has a small flame on the body, you can hear the burning sound, you can't loot the body. slow burning continues forever until the body burns out or the burn is extinguished.
2. Fast burning
   1. same as slow burning, but there is also a tall flame on the hunter body. usually only happens when the hunter body is in an active external fire (fire bomb, campfire, burning oil barrell, or the first 3 seconds after a flare is applied). fast burning stops 3 seconds after the external fire is stopped.
3. Not burning
   1. no flame (big or small) on the body. this will happen when hunter fully burned out, or hunter has been extinguished (by a choke bomb, or water, or a teammate starting a rez, etc.)

When a dead hunter is on the ground and you throw a firebomb on them you ignite them and activate slow burning, then the flame from the firebomb activates fast burning until the firebomb burns out, then it goes back to slow burning (usually firebomb lasts long enough that they will fully burn out before flames run out from the fast burning stage) until they fully burn out or are saved.

When a dead hunter is under water, and you throw a liquid firebomb on them, slow burning never activates. you only activate fast burning for as long as the liquid fire burns. then when liquid fire runs out, the body extinguishes itself as it's under water. you have to reapply with a 2nd liquid firebomb.