None, as long as you don't mind the volitans eating the dwarf.
You may be able to pull it off if you obtain a juvie volitans and a larger dwarf (I recommend a fuzzy) and let them grow up together, but personally, I wouldn't try it. A better fit would be a large-bodied Pterois species (volitans, russelii) with a medium-bodied Pterois species (antennata, mombassae, radiata). Of the medium-bodied lions, the antennata has the largest mouth.
Whatever you decide, it's all about "size differential" and risk tolerance...if one fish thinks it can swallow the other, it will try. We've never had any of our Scorpaeniformes eat another one, but we're pretty conservative in mixing them.