A differential clock has the advantage of twice the SNR and much better common mode rejection and low jitter with controlled balanced impedance.
A two phase clock that simply uses an inverter with same rising edge has some skew whereas a differential clock has less skew since delays are similar.
Actually this just 1 phase inverted and should not be confused with a 90 deg shift or quadrature 2 phase clock used for other applications.
If asynch. clock is used , it must be synch'd by receiver 2x clock , so async clock must be <= 1/2fmax or >= 2 periods wide.
That has nothing to do with single or differential.