I'm quite sure I have missed something fundamental, but I am not seeing the ignite spreading being added in the way described in the original post.
I performed a very simple test on the garrison target dummy of dropping PC, pyroblasting it then 2x IB to stack the ignite.
I made sure to only pass ignite to the one dummy, and I removed some of my gear to avoid any potential complications, just wearing 2 part Tier.
From what I have read here, I would have expected the target dummy to have an ignite value that reflected the initial pyro ignite passed from IB, then to have that added to, by the second IB, a value that comprises of (Pyro Ignite + First IB ignite) - Ignite tick 1 - Ignite tick 2.
A screenshot of the resulting combat log is here:
http://i.imgur.com/ocgPU5j.png" target="_blank
What I believe should have happened was this:
Mastery 17.22%
Pyro = 11701 Remaining PC Ignite = 11701*0.1722 = 2014.9
IB = 4248 Remaining PC Ignite = 2014.9 + (4248*0.1722) = 2746.4
Ignite of pyro only passed to target dummy = 2014.9
Tick (PC) = 458 Remaining PC ignite = 2746.4 - 458 = 2288.4
Tick (PC) = 458 Remaining PC ignite = 2248.4 - 458 = 1830.4
2nd IB = 4248 Remaining PC ignite = 1830.4 + (4248*0.1722) = 2561.9
Remaining Ignite of pyro and 1st IB only passed to target dummy = 1830.4
Total ignite passed to target dummy = 2014.9 + 1830.4 = 3845.3
However from the screenshot you can see the total ignites on the target dummy were:
403
384
384
384
384
384
384
Total = 2707
I can only conclude from this that ignite stacking does not work, but you chaps seem to be using it successfully. What am I missing?