Design procedures for constructing complementary binary sequences and co-operative binary sequences are available in the literature. In the paper an algorithm for constructing pairs of ternary sequences which are co-operative not only in the autocorrelation domain but also in the signal domain (and hence called ‘doubly’ co-operative) is presented. The two signals can be transmitted as if they are a single sequence with energy efficiency of 100% and with appreciably large co-operative merit factor. Good co-operative pairs of sequences up to length 29 are reported.