Oh! That's neat - can you do that "on the fly"? Has anyone measured the performance impact?
I edited the comment. Some docs have io 10 as CS, some io 11.
I think that if the flash is wired QIO you can't use the QIO pins.
And you can connect an SPI device to the bus but the CS pin of flash is controlled by SDK.
Note that some boards and modules (ESP-12ED, NodeMCU 1.0) also break out pins 9 and 11. These may be used as IO if flash chip works in DIO mode (as opposed to QIO, which is the default one).
This mode shares the SPI pins with the controller that reads the program code from flash and is controlled by a hardware arbiter (the flash has always higher priority). For this mode the CS will be controlled by hardware as you can’t handle the CS line with a GPIO, you never actually know when the arbiter is going to grant you access to the bus so you must let it handle CS automatically.