  For any compact connected Lie group $G$, we study the Hamiltonian sum of two
compact Hamiltonian group $G$-manifolds $(X^+,\omega^+,\mu^+)$ and
$(X^-,\omega^-,\mu^-)$ with a common codimension 2 Hamiltonian submanifold $Z$
of the opposite equivariant Euler classes of the normal bundles. We establish
that the symplectic reduction of the Hamiltonian sum agrees with the symplectic
sum of the reduced symplectic manifolds. We also compare the equivariant first
Chern class of the Hamiltonian sum with the equivariant first Chern classes of
$X^\pm$.
