Key points are not available for this paper at this time.
David L. Borchers, Stephen T. Buckland, Paul W. Goedhart, Elizabeth D. Clarke, Sharon L. Hedley, Horvitz-Thompson Estimators for Double-Platform Line Transect Surveys, Biometrics, Vol. 54, No. 4 (Dec., 1998), pp. 1221-1237
Borchers et al. (Tue,) studied this question.