Hi Rob,
First, see correction to my previous post.
I cannot tell you what the designers had in mind at that moment, but keep in mind the amount of pins available for multi-function purposes. There are trade-offs in design...for example, you could not use the secondary I2S if you're using SPI. Another one is that SCLK is only an input so it cannot be used as output. If you think about it, you could also have your setup working as master mode using DOUT as S_WCLK. But this requires to have the first I2S chip ignore the S_WCLK switching activity in its RX pin connected to DOUT of AIC device.
Again, Figure 2-50 of the reference guide will provide all the available configuration possible.
Feel free to ask us questions. We'll try to answer them as soon as possible.
Regards,
J-