  We present a class of exactly solvable quantum spin models which consist of
two Heisenberg-subsystems coupled via a long-range Lieb-Mattis interaction. The
total system is exactly solvable whenever the individual subsystems are
solvable and allows to study the effects of frustration. We consider (i) the
antiferromagnetic linear chain and (ii) the Lieb-Mattis antiferromagnet for the
subsystem-Hamiltonians and present (i) the complete ground-state phase diagram
and (ii) the full thermodynamic phase diagram. We find a novel phase which
exhibits {\it order from disorder} phenomena.
