Series regulator noise levels
This is what I get out of the setup you envision:

SMPS -> VReg* -> SMPS

And your asking about the rail with the * on it. 

In this case, the last SMPS will affect the regulation of the Vreg because it is switching current and because the Vreg has source impedance, and cannot respond infinitely fast to regulate the voltage.

The question then becomes, how much does it affect regulation? This information can be found in good voltage regulation products as they display a graph that shows ripple rejection vs frequency. The SMPS has a switching frequency, and the voltage regulator will reject the ripple and the amount in dB can be found and the amount of rejection calculated. 

In most cases, I would find it confusing to place an SMPS after a regulator because


Voltage regulators generally have higher losses
SMPS usually have wide voltage inputs