We investigate the two-neutron correlations in 6He and 11Li nuclei through the density distributions of two valence neutrons. In this work, we use the two-neutron wave functions for halo nuclei within the extended three-cluster model assuming the core+n+n structures, where the internal correlations in the core nucleus is taken into account. For the 6He nucleus, the mixing of higher partial waves increases the spin singlet states slightly and enhances the di-neutron correlations. For the 11Li nucleus, a large s-wave mixing suppressed to form the spin triplet states and the di-neutron correlations are dramatically enhanced by the large amount of the singlet states.