Mammalian embryos can be spilt usually up until you reach the
blastocyst stage of development. At that point, the embryo has
developed a structure and although there still are cells that can
contribute to an entire organism (inner cell mass cells) the mechanical
damage done by splitting normally precludes further development of
either half of the embryo.
Embryos can be split and then recombined to produce only one
offspring. In fact, two or more (there is an upper limit) seperate
embryos can be combined together (aggregated) so that only a single
birth results. This offspring is called a chimera. It looks and acts
normal in all respects except that it is really two or more animals in
one. You can see the effect of chimerism if you combine embryos that
have different phenotypes (like one comes from a black hair strain and
the other comes from a red hair strain - in this case the chimera will
have blotches of red and black hair).
How does that sound?