A system in a package (SIP) has a first die with a first internal voltage level, first die-to-die output circuitry, first die-to-die input circuitry, and first internal logic and a second die with a second internal voltage level, second die-to-die output circuitry, second die-to-die input circuitry, and second internal logic. A first signal is provided to the second internal logic via the first die-to-die output circuitry and the second die-to-die input circuitry, wherein each of the first die-to-die output circuitry and second die-to-die input circuitry selectively level shift the first signal based on the first and second internal voltage levels. A second signal is provided to the first internal logic via the second die-to-die output circuitry and the first die-to-die input circuitry, wherein each of the second die-to-die output circuitry and first die-to-die input circuitry selectively level shift the second signal based on the first and second internal voltage levels.