A Two Parameter Four Neutrino Mixing Model with an Exchange Symmetry of   the Mass Doublet Neutrinos
  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. 
E.M. Lipmanov