Yes, you can, and you shouldn't need a release agent.  You can just use each half separate, then glue them together when they're dry.  Or almost dry.

Lay the two halves face up.  Wet your paper strips/squares and triangles but don't add any adhesive at all. [Note: for a mold of this kind, with very tight curves, your paper will have to be in pretty small pieces -- those strips just won't work.]  Line your mold halves with the wet paper, being careful to overlap and press
into the shapes of mold carefully. This is one reason that you don't need a release agent, the paper is it.

Then start adding your paper pieces that have adhesive on them. The strips should not be soaking wet with glue, just brush the glue on both sides. Press and smooth each paper into place.  For some reason, cut edges just don't lie down and seal properly, so make sure all your edges are torn. Discard factory edges, they're the worst.

The number of layers depends on what kind of paper you're using (the 'hardness' and the thickness) and the size of the mold you're using. There should probably be at least four or five complete layers, maybe more.  It's perfectly fine (and maybe best) to leave the raw edges sticking up, but don't fold them over onto the flat edge of the mold.  Let dry.

Plaster is the only mold material I know that will help the papier mache dry. As in ceramics, the porous plaster will absorb moisture from the casting.  PM shrinks as it dries, and it will pull free of the plaster mold in most places, making it easy to remove.

Don't heat the PM casting, or it will warp, and you really don't want it to warp, as you will be needing to fit the two halves together.  But to speed up drying, you can place the casting in front of a small fan set on Low.  Just the air moving over the PM will help it to dry.  A sunny windowsill is good, too.

I've only done this once, but I let the castings dry too much, and they warped just enough so they were fighting the adhesive and I couldn't get them back together properly . Fortunately, I was using a methyl cellulose adhesive, and I was able to spray it lightly with some water to soften it so I could manually reshape it. (If I had been using white (vinyl) craft glue, I don't think this would have been possible.)  So, I would advise checking the casting frequently when it's almost dry.  I think it should be holding it's form well, but still be slightly pliable.

Trim off all the paper that is projecting beyond the rim of the mold.  The easiest way to do this is to use a pencil or marker and draw the line, then remove the casting from the mold and gently and carefully trim it with scissors.

Fit the pieces together and fasten with strips of paper and your adhesive.  Make the strips long enough to grip securely.

Check the surface of your casting. Any obvious place where you didn't press the very first (glueless) layer into the mold may need to be filled in.

USING PULP:  If you want to use pulped paper, you'll still have to line the mold with plain paper with no adhesive. Not only will it act as a mold release, if you do it carefully, you will have a much smoother finished casting on the outside.

Don't make your pulp/adhesive mix too wet.  Make it just moist enough to press completely into the mold crevices.  Wet, sloppy pulp will take a long time to dry, and is likely to shrink and warp more.  Experience will be the best teacher in this regard.

Once your glueless paper liner is in the mold, start pressing your pulp into the deepest crevices first.  Again, the thickness of your casting material will depend on how large the mold is.  A quarter of an inch (5 or 6mm) would probably be sufficient for most things, but you might have to make it as thick as 1/2" (10-12mm).  This should be enough for a really large casting.  If you're using white (vinyl) craft glue, you might want to do your 1/2" (10-12mm) casting in a fewer, thinner layers.

There is no need to completely fill the mold. It's a waste of materials and will take a very long time to dry.  If you're using a white (vinyl) craft glue, the inside will probably never dry, which will lead to mold problems after you decorate and seal it.  If you absolutely NEED to make a solid object (and I can't imagine why), you could either build up the casting one layer at a time and let each one dry completely before adding the next, or use an organic glue like methyl cellulose, and fill the entire mold all at once.  With MC, the moisture will be able to gradually wick through the casting to the surface, and dry.  You can't do this with white (vinyl) craft glue. Once the casting is firm and will hold it's shape (not dry yet), you'll probably have to remove it from the mold to continue drying it.

Some other organic adhesives that will allow moisture to migrate out of the casting are wheat flour, rice flour and potato flours.  But with these materials, always keep in mind that if the project is not sealed well, the food-type nature of the adhesive may attract insects and rodents (not to mention voracious Belgian Tervurens, but that's an ugly story).

If you think you will need to weight your casting, don't forget to add it BEFORE you connect the two sides together.  Weights comprised of many pieces (lead or steel shot, sand, beans, etc) should be bagged and sealed.

The more pieces you do, the more you will learn what works and what doesn't.

And be sure to have fun!

Sue