  We derive a system of covariant single-time equations for a two-body bound
state in a model of scalar fields $\phi_1$ and $\phi_2$ interacting via
exchange of another scalar field $\chi$. The derivation of the system of
equations follows from the Haag expansion. The equations are linear integral
equations that are explicitly symmetric in the masses, $m_1$ and $m_2$, of the
scalar fields, $\phi_1$ and $\phi_2$. We present an approximate analytic
formula for the mass eigenvalue of the ground state and give numerical results
for the amplitudes for a choice of constituent and exchanged particle masses.
