  We present model results for the two-halo-neutron correlation functions,
$C_{nn}$, for the dissociation process of light exotic nuclei modelled as two
neutrons and a core. A minimum is predicted for $C_{nn}$ as a function of the
relative momentum of the two neutrons, $p_{nn}$, due to the coherence of the
neutrons in the halo and final state interaction. Studying the systems
$^{14}$Be, $^{11}$Li and $^{6}$He within this model, we show that the numerical
asymptotic limit, $C_{nn}\to 1$, occurs only for $p_{nn}\gtrsim$ 400 MeV$/c$,
while such limit is reached for much lower values of $p_{nn}$ in an independent
particle model as the one used in the analysis of recent experimental data. Our
model is consistent with data once the experimental correlation function is
appropriately normalized.
