Find your parting line
To make my parting line using foam core I cut a circle slightly smaller then the diameter.
Fit it on your parting line.
Fill in gaps with oil clay.
Create a ridge all the way around your parting line to key the two pieces of latex together. Use additional keys to orient the Pates together and for keying the plaster mother molds together.
First coat is thin for detail.
Create a flange that covers past the ridge.
If you can store your brush right in your latex that's the best.
Let each coat dry. The white parts are not dry yet.
After several coats, incorporate sponges to key the latex into the mother mold.
Many coats. Completely dry.
Now for the mother mold.
Create a flange larger then your latex flange.
Flip and clean up.
Final coat on side two.
Not very happy the latex flanges curled.
I clamped the curled latex as I made the second mother mold.
Drilled for screws to hold the finished pieces together.