after poking around your scene,
i killed the dens emission on the second emitter, and transfered that value to the first emitter,
id personally use render layer overrides on the emitter and separate my heat and density and diffuse light ect ect that way,
then each element you want has its own render layer, and possibly different attribute values accordingly.
there was a tutorial ( id love to get ahold of ) a while ago going around with an ice cream truck explosion, where the author explained this method alot better than i can here,
( if you find that tutorial id love a copy, it could have been a WayneHollingsworth tut , im not sure tbh )
also, id def increase the sub steps and the solver quality, unless you just dialed it all down for the file share.
hope this helps