Hey:
So what type of lion? the large bodied lions like P Volitans, russells, miles will easy fill a nice 100 gal+ tank, The mid bodied lions like antennata, radiata, mobassae maybe a 75 to a 100, and dwarfs easily undr 50gal.
Also what type of trigger? Planktonic feeding ones (like pinktails, niger, blue cheeks) are pretty mellow and won't shred a lion. My minimum would be 75gal for one, but bigger is much better.
I would say over all a 100 or 125 would be the smallest for this mixture (lionfish dependent-small lions wi ould drop to a 75)
frank