  In the archetypal strongly correlated electron superconductor CeCu$_2$Si$_2$
and its Ge-substituted alloys CeCu$_2$(Si$_{1-x}$Ge$_{x}$)$_2$ two quantum
phase transitions -- one magnetic and one of so far unknown origin -- can be
crossed as a function of pressure \cite{Yuan 2003a}. We examine the associated
anomalous normal state by detailed measurements of the low temperature
resistivity ($\rho$) power law exponent $\alpha$. At the lower critical point
(at $p_{c1}$, $1\leq\alpha\leq 1.5$) $\alpha$ depends strongly on Ge
concentration $x$ and thereby on disorder level, consistent with a
Hlubina-Rice-Rosch scenario of critical scattering off antiferromagnetic
fluctuations. By contrast, $\alpha$ is independent of $x$ at the upper quantum
phase transition (at $p_{c2}$, $\alpha\simeq 1$), suggesting critical
scattering from local or Q=0 modes, in agreement with a density/valence
fluctuation approach.
