Thats a pretty heafty question. I had to go to collage for 3 years to answer that, lol. I mainly use ultra-cal 30, or fiberglass, or hydrocal, or silicon depending on what im trying to make. If im making a latex mask then ultracal 30 is the way to go in my opinion. As far as how to make it 2 peices- the answer is here and is to much to discribe.
well most people and myself on here use ultracal 30 its a gysum product its cheaper than silicon
but man it weights a ton on a big mold I used it for my chest and legs pieces which you can see at
my suit progress thread the link is my signature.