Two QSPI devices share the same bus in QSPI dual-stacked mode to double the maximum addressable flash memory storage for the application. This mode also reduces the boot interface I/O pin count because the bus is shared and only one additional interface pin is needed for the flash select. In this mode, only the lower QSPI addressable flash memory space can be used for boot and the throughput remains the same as it is in the QSPI single-device mode.
An example of the dual-stacked QSPI setup is with the 32-bit addressing mode is shown in the following figure.
Figure 1. Dual-Stacked Quad SPI Interface Example