Adding capacity to an SD card means either increasing the density of the flash chip or using more flash chips. In this case the op was referring to doing the later strategy. In order for additional dies to not alter the power characteristics of the card, the controller chip would have to have some sort of active power management built in power off dies that aren't in use.
Idle energy use for a typical SD card is well under 1mA. Even though the OP was suggesting something a bit more sophisticated than 4 entire separate cards inside one, it would work just fine.
