But it seems like the dac only needs two pins to communicate via SPI... because it doesn't need to send data back to the Arduino. So pin 12 on the Arduino doesn't need to be connected to anything.
True, you don't have to use MISO.
I can't tell if it's needed for the smaller single dac chip I'm using.
The single version still has an LDAC pin, so yes it would be needed.
Unfortunately LDAC is an active LOW latch enable so you can't just hook it up to SS on the SPI interface as SS will go low for the entire frame. You can with the SR because it's a rising-edge clock, but not the DAC.
You need to organise a low pulse (min 100nS) on the rising edge of SS and feed that to LDAC.