How do I daisy-chain multiple potentiometers? How does the serial interface work for multiple AD8403's when there are still only two address bits? Are there multiple /CS lines, one for each individual chip?

The address bits are for addressing multi-channels within a device only. For multiple device operation, you will need to daisy-chain them. For example, daisy-chaining two quad-channel AD8403 gives a total of 8 channels. You can program any combination of two channels (in different packages) at the same time with the 1st device's SDO pin tied to the 2nd device's SDI pin with a 10k pull-up resistor. Then, you would send 20 bits into the SDI pin of the 1st device while keeping the /CS of both devices low. The 1st 10 bits go to Device 2 and the 2nd 10-bits go to Device 1. When /CS is pulled high, both channels will be updated.