Improved Randomized Response Technique for Two Sensitive Attributes
  We proposed new and more efficient estimators for estimating population proportion of respondents belonging to two related sensitive attributes in survey sampling by extending the work of Mangat (1994). Our proposed estimators are more efficient than Lee et al (2013) simple and crossed model estimators as the population proportion of possessing the sensitive attribute increases. 
Olusegun S. Ewemooje and Godwin N. Amahia
stat.ME