First I would mold the teeth and mandibles in silicone, cast up a pair and make sure everything sits right in the sculpt. Then make a mold wall with keys along the crown and side of the face (sort of along the line a biomask doesn't cover). Mold the front allow ultracal or resin to cure and then remove the mold wall and mold the back. When casting in latex I'd start painting it with a brush into both shells until it's thick enough to join mold-halves, strap them together and do multiple layers of latex along crease, and then foam it (outside toxic fumes!)while in the mold. Separate the mold and you've theoretically got a mask
Cool I have done single mold but never a double mold and I am nervous like there is no tomorrow. I have watched some tutorials on mask making keys etc. but never did because the mask wasn't this big.so uhhhh so we will see