  The two-loop chiral measure for superstring theories compactified on $\bZ_2$
reflection orbifolds is constructed from first principles for even spin
structures. This is achieved by a careful implementation of the chiral
splitting procedure in the twisted sectors and the identification of a subtle
worldsheet supersymmetric and supermoduli dependent shift in the Prym period.
The construction is generalized to compactifications which involve more general
NS backgrounds preserving worldsheet supersymmetry. The measures are
unambiguous and independent of the gauge slice.
  Two applications are presented, both to superstring compactifications where 4
dimensions are $\bZ_2$-twisted and where the GSO projection involves a chiral
summation over spin structures. The first is an orbifold by a single
$\bZ_2$-twist; here, orbifolding reproduces a supersymmetric theory and it is
shown that its cosmological constant indeed vanishes. The second model is of
the type proposed by Kachru-Kumar-Silverstein and additionally imposes a
$\bZ_2$-twist by the parity of worldsheet fermion number; it is shown here that
the corresponding cosmological constant does not vanish pointwise on moduli
space.
