  We use Langer's approach to calculate the reaction rate of a system of two
(classical) spins interacting via the exchange coupling $J$ in a magnetic field
$H$, with uniaxial anisotropy of constant $K$.
  We find a particular value of the exchange coupling, that is $j\equiv J/K =
j_c\equiv 1-h^2$, where $h\equiv H/2K$, which separates two regimes
corresponding to a two-stage and one-stage switching.
  For $j\gg j_c$ the N\'eel-Brown result for the one-spin problem is recovered.
