SPI broadcast: talk to many slaves
First, SPI was not designed for broadcast transmissions.
You can do this, with few caveats:


The MISO lines are not connected together. You will get collisions if they are connected.
All the slaves are similar devices, weird stuff can happen when the wrong opcode is sent to a device.


You should use a separate CS line for each slave, and not tie them all together, just in case.