The paper  of Seidel in 1976 is commonly cited when peoples mention two-graphs, but Taylor wrote down the concept of regular two-graph, which is stronger than two-graph in his PhD thesis  in 1971, in where he mentioned that regular two-graph was due to his advisor D.G.Higman. Seidel himself also pointed out in  that the concept of regular two-graph is due to Higman.
Now if I want to mention regular two-graph, then certainly Higman is the original contributor. But if I want to mention two-graphs in my paper, whom should I identify as the original contributor of the concept of two-graph?
 J.J.Seidel (1976)-A survey of two-graphs, Coll. Intern. Teorie Combin., Atti dei convegni Lincei 17, Roma, pp.481-511.
 D.E.TAYLOR (1971)-Some topics in the theory of finite groups, Ph.D. thesis, Univ. Oxford.
 J.J.Seidel (1992)-More about two-graphs, in Fourth Czechoslovakian Symposium on Combinatorics, Graphs and Complexity (J. Nesetril and M. Fiedler, Eds.), pp. 297-308.