To release silicone rubber from silicone rubber, use a release agent that does not contain a significant amount of silicone oil.
Mann’s Ease Release® 200 was made especially for releasing silicone rubber from silicone rubber and other surfaces. It goes on as a light mist coating and will not interfere with surface detail.
Option: you can also use petroleum jelly thinned with a solvent such as mineral spirits. Make sure you don’t leave brush strokes on the cured rubber’s surface as they will be reflected in the mold.