  We consider the interaction potential between two chiral rod-like colloids
which consist of a thin cylindrical backbone decorated with a helical charge
distribution on the cylinder surface. For sufficiently slender coiled rods a
simple scaling expression is derived which links the chiral `twisting'
potential to the intrinsic properties of the particles such as the coil pitch,
charge density and electrostatic screening parameter. To predict the behavior
of the macroscopic cholesteric pitch we invoke a simple second-virial theory
generalized for weakly twisted director fields. While the handedness of the
cholesteric phase for weakly coiled rods is always commensurate with that of
the internal coil, more strongly coiled rods display cholesteric order with
opposite handedness. The correlation between the symmetry of the microscopic
helix and the macroscopic cholesteric director field is quantified in detail.
Mixing helices with sufficiently disparate lengths and coil pitches gives rise
to a demixing of the uniform cholesteric phase into two fractions with a
different macroscopic pitch. Our findings are consistent with experimental
results and could be helpful in interpreting experimental observations in
systems of cellulose and chitin microfibers, DNA and {\em fd} virus rods.
