The nested birdcage design is useful to develop dual tuned volume transceiver RF coils. Despite its apparently simple design, coupling among coils affects the resonance frequencies, making its practical realization cumbersome. FEM simulations were first validated by workbench measurements for a specific nested double-birdcage suitable for 1H/23Na MRI at 2.35T. Then were used to study the isolation and RF efficiency for a set of different geometrical parameters. We demonstrate that an optimized nested design is obtained if the disposition of the birdcages rugs, lengths and shield diameter are carefully taken into account.