  In a model with four neutrino mass eigenstates grouping in two narrow
doublets the condition of maximal mixing of the doublet neutrino components
leads to a simple four-neutrino mixing matrix which is related to two connected
effective two-neutrino mixing patterns. As a result, large values of the
neutrino oscillation amplitudes for both the solar and atmospheric neutrino
anomalies appear quite naturally if the LSND data are accepted.
