In this paper, we address the problem of timing speculation for multi-threaded workloads executing on a multi-core processor. Our approach is based on a new observation - - heterogeneity in path sensitization delays across different threads in multi-threaded programs. Leveraging this heterogeneity, we propose Synergistic Timing Speculation (SynTS) to jointly optimize the energy and execution time of multithreaded applications. In particular, SynTS uses a sampling based online error probability estimation technique, coupled with a polynomial time algorithm, to optimally determine the voltage, frequency and the amount of timing speculation for each thread. Our experimental evaluations, based on detailed cross-layer simulations, demonstrate that SynTS reduces energy delay product by up to 21%, compared to existing timing speculation schemes.