Don't do it.
For one thing, aggression or territorial behavior has nothing to do with feeding behavior...at all...not even a little bit. Feeding behavior is something else altogether, and is a separate consideration completely. This is aside from the fact that lionfish are not normally territorial towards conspecifics let alone heterospecifics. In other words, they could care less about a few clowns in their vicinity. This is most of the time...nothing is all the time.
I've been keeping marine fish for several decades and change, and by this time I've seen just about everything, including Lions eating fish two thirds their own length, and groupers swallowing Lions that actually were their own length...the list goes on....
For another thing, these "dwarf" lions generally get big enough to easily swallow an occelaris clown at full adult size.
Also, these fish are nocturnal predators, and will cruise the entire tank looking for unwary prey, so territory doesn't come into play...you'll lose your clowns one night at 2:00 am 6 months from now.
Also, sometimes a fish can't quite manage swallowing another, but kills it in the effort...that's always a good one. :|
Bottom line, I would never do this as I know it's just a matter of time.
Jim