Dual threshold voltage (Vth) design is a common method for reducing leakage power in above-threshold circuits. This research shows that it is also effective in reducing energy per cycle of sub-threshold circuits. We first study the single-Vth design theoretically and by simulations, and find that the energy per cycle is independent of threshold voltage. However, in a dual-Vth design, the energy per cycle depends on both threshold voltage and supply voltage. We propose a framework to further reduce energy per cycle below what is possible with a single Vth. Given a nominal value for Vth, we determine an optimal supply voltage Vdd and an optimal higher Vth. Application to a 32-bit ripple carry adder shows energy saving of 29% over the single-Vth lowest energy.