There's no way to do that directly, you'll have to cheat with careful positionning and hiding transition between the two with clothes / accessories or postwork
You could try something like this:
load both in zero position
hide G8 head (and probably neck) and dragon's body
scale dragon so that the head size is coherent with G8 body and position it in the right place
parent dragon to G8 chest or neck so it roughly follows G8 poses
use whatever morphs you have in both figures to make it look better
Comments
There's no way to do that directly, you'll have to cheat with careful positionning and hiding transition between the two with clothes / accessories or postwork
You could try something like this: