1) What is the recommended approach when using only two slaves? We have two /SS pins available from the host MCU. It looks like we should connect the first slave select to /MSS, the second to SSA0, and fix SSA1.
2) DS table 18, suggests that MI is driven even when the chip is not selected; if we want to have another device communicating on the line, we need to power down this chip to avoid contention on this line. Can you confirm this is the case?
The MSS signal is the input to the demultiplexer on the ADuM3154. The SSA0 and SSA1 pins are the selects for that demultiplexer. So the MSS signal is steered to any one of the SS0/1/2/3 pins depending on the states of the SSA0 and SSA1 pins.
The typical way to operate the part would be to set the SSA0 and SSA1 pins, then send the chip select signal on MSS to the selected slave.
Note the SS0/1/2/3 pins go to HiZ when they are not selected. This means there should be pull ups or pull down on the lines depending on the polarity of the chip select signal.
The ADuM3154 was designed to be flexible and work with the different implementations of SPI. That means the MI pin is driven with the MSS signal high or low. For example, if the MSS signal is high, the MI pin will still be driven to whatever state is on the SO pin.