Hi coilbook,
Multiple sources works fine, what matter are the resulting channels in the volume.
Each channel can be created from different sources, Octane do not know or check.
To investigate such problem, it's better to enable only 1 channel and disable everything else.
So first, in the Octane Volume modifer, disable the Absorption channel and the Scatter channel.
As we can see in the Phoenix simulation window, the temperature has values from 300 to 2000.
For the emission ramp, volume ramp texture : set the Max. grid value to 2000.
Then it's more easy to adjust the other values (power, scales..) to see the fire.
Do the same to see the smoke only in the scatter channel, by disableling the emission.
Note that the smoke has values from 0 to 1, so set the Max. grid value to 1..
Then both should be visible, as in the attache scene.
It seems that setting the Absorption to Smoke and emission to temperature makes the emission disappear, so keep the Absorption disabled..
I will check if that's a bug, and try to fix the orbx export that doesn't works correctly for this case..
I hope that helps,
Regards