SPI0 Overlap with multiple slaves

### Basic Infos

SPI0 Overlap is now #1062 supported, awesome!
I need to address more than one SPI Slave....

#### Hardware
Hardware: ESP-12-E
Core Version: 2.1.0-rc2

### Description
The implementation suggests that there is a possibility to use custom CS thus being able to use more than one SPI slave:
https://github.com/esp8266/Arduino/blob/master/libraries/SPI/SPI.cpp#L129

Question, is this true or the above code does not make sense?

### Settings in IDE

Module: Generic ESP8266 Module
Flash Size: 4MB/1MB
CPU Frequency: 80Mhz
Flash Mode: qio
Flash Frequency: 40Mhz
Upload Using: OTA / SERIAL
Reset Method: ?ck

### Sketch

```cpp
case SPI_PINS_HSPI_OVERLAP:
if (use) {
...
}
else {
if (useHwCs) {
pinMode(SPI_OVERLAP_SS, INPUT);
SPI1P |= SPIPCS1DIS | SPIPCS0DIS | SPIPCS2DIS;
SPI1U &= ~(SPIUCSSETUP | SPIUCSHOLD);
}
}
```