  We calculate at two-loop order the complex-valued scattering amplitude
related to the twice-iterated scalar-isovector boson-exchange between nucleons.
In comparison to the once-iterated boson-exchange amplitude it shows less
dependence on the scattering angle. We calculate also the iteration of the
(static) irreducible one-loop potential with the one-boson exchange and find
similar features. Together with the irreducible three-boson exchange potentials
and the two-boson exchange potentials with vertex corrections, which are also
evaluated analytically, our results comprise all nonrelativistic contributions
from scalar-isovector boson-exchange at one- and two-loop order. The applied
methods can be straightforwardly adopted to the pseudoscalar pion with its
spin- and momentum-dependent couplings to the nucleon.
