>> The BCM283x SoCs at the heart of every Pi has two MMC controllers. One
of these connects to the SD card slot that the Pi boots and runs its OS
from. The other has largely been ignored, but comes out on the HAT connector
as alternate functions on GPIOs 22-27. Recent kernel and boot blob
changes have enabled these pins to be mapped to the MMC controller that
also supports SDIO devices.
>> SDIO is the SD host/eMMC interface on the Raspberry Pi. SD host signals are normally used for the microSD slot.These pins are "SD host" on Alt0 and "eMMC" on Alt3.
>> The SD cards are primaraly a SPI device, as such you technically could
parallel anouther device as long as you use a seperate chip select line
ut this would then require special driver to know what to use for it.
Sometimes it is the hardest to find definitive statements about obvious things. ajlitt has done an excellent job documenting all there is to know about the MMC controller and the secondary SDIO interface. The facts have changed over the past time so I initially found some contradictions.
Looking forward to Jacksons PCBs and ESP 12F modules from abroad.