Item from Mathematical Reviews (American Mathematical Society)--access their stable url through the AMS database via an institutional library:

MR1811997 (2001j:91103) Roberts, Fred S.; Sheng, Li How hard is it to determine if a graph has a 2-role assignment? Networks 37 (2001), no. 2, 67--73. (Reviewer: Sandra L. Arlinghaus) 91D30 (05C85 05C90)