  In the actively coupled ($\mathcal{AC}$) pair of waveguides, the growth of
small perturbations is saturated by the focussing nonlinearity that couples the
linearly growing to the linearly damped mode. On the other hand, in the
$\mathcal{PT}$-symmetric coupler, the focussing nonlinearity promotes the
blowup of stationary light beams. The purpose of this study is to compare the
nonlinear dynamics and explain the opposite effect of the same nonlinearity in
the two systems. We show that while the blowup regimes are stable in the
$\mathcal{PT}$-symmetric pair of waveguides, they are unstable and hence cannot
be observed in the $\mathcal{AC}$-dimer.
