I used to make masks for a couple of years. My technique was to sculpt water based clay onto a polystyrene wig former which was attached to the neck and shoulders of a life cast. When the sculpture was finished I would cover it with hard dental plaster. Once that had hardened I turned the whole thing over and removed the shoulders which gave access to the base of the wig former. I then poured a solvent onto that (aceton, I think) which dissolved it and gave me space to dig out the clay. Once done I had a seamless, one piece mould.
Doing it the way I did it negated the need to create a two piece mould. Later on I made a fibreglass head and shoulders that I cut into sections, and bolted together. Again, it allowed me to make a one piece mould. Hope that is of help. I'm happy to answer any other questions about my method of latex mask making.