The interface uses 2 clock signals: this provides a reasonable EMI reduction and it also extend the protocol.

The 2 clock wires are required to encode the 4 edges of a full transaction in normal/usual mode. The signals follows the traditional Gray counting sequence:

00
01
11
10

and the whole transaction is ended when both signals are back to 0.

Swapping the sigals corresponds to counting in reverse:

00
10
11
01

This signals that the transaction contains a control or status word.

There is a rule that only one wire can change at a time and each front updates the state of a sort of FSM.

Naming the clock signals A and B, we get a sequence of these letters and the sequence ends when the number of As and Bs are both even.

Thus we get these 4 basic sequences:

AA : reset AC register  (set to current value ?)
BB : Update link status (9 bits?) 
ABAB : Data transaction (18 bits)
BABA : Control transaction

Another way to visualise is with a binary decision tree: 

You get the idea: the protocol is extensible and we start with a few good cases that are easy to use without too much complexity.