As Tessel 2 has two cores, which one is the master? The reason that it is good to switch off one of the core when it is not needed.
Secondly, what is the interface between the two cores? Shared memory? Pins? SPI?
SAMD21 has the ability to switch off the power to the MT7620n. SAMD21 itself can go into a low power sleep mode.
They communicate over SPI + two GPIO pins. The MT7620n is SPI master.
See https://github.com/tessel/v2-firmware/blob/master/README.md for more details on how this is implemented. The schematic may also interest you.