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[Dave Smith]

Silicone will only stick to anything really porous, or more silicone. It has been done before, even with a fiberglass mother mold, but making the mother mold (shell mold) porous is tricky. People have also added a million nails to the inside of the blanket mold to get a mechanical bond, but it only holds for a while. First, are you really sure you need them to bond? There are lot's ways to get the silicone to "snap" in place on the mother mold. Second, if you are sure, consider going to a non-silicone elastomer such as urethane rubber (even an rtv urethane) and then using a rigid urethane mother mold.   Dave  PS: you can also make a soft silicone blanket, with a more rigid silicone shell.

[Roadkill]

You need some texture on the silicon, smooth it first, then make some small buttons of silicon buy spraying some release in a large drinking straw, pour some silicon in it, and then let it set, Cut the cured silicon into short pieces, like 1/4" and then use a little mixed up silicon ot attach them to the smooth silicon mold. Then make you mother mold and make sure you get these little snaps coated really good, and they wil hold the mold into the mother mold.
It is way better to have the two molds seperate as you will have a tough time getting the mold off if there is any undercutting on the limb. If it is not exactly smooth, it still will not want to come out of the mold. Having the silicon mold flexable is perfect.
Make sure the rest of the silcon mold is smooth on the outside or you may not get the mother mold off.