I hook them the same when using them dead and alive. Just one hook through the mouth and one through the back fin. I use 5/0 – 7/0. It’s also important to use a wire trace otherwise you’re most likely going to get bitten off. You also need a small ball sinker to keep the mullet down, and set it about 1 meter below the surface.
Barracouta are pretty stupid though. When I ran out of mullet at Kaikoura I put a peice of one of the barracouta I’d caught under a float and hooked a barracouta. Tried a spotty under a float another time but it just circled it a couple times and swam off. They pretty much always go for the head end so the second hook is more to hold your bait on especially when casting.