  Gamma-ray bursts (GRBs) have been widely argued to originate from binary
compact object mergers or core collapses of massive stars. Jets from these
systems may have two components, an inner, narrow sub-jet and an outer, wider
sub-jet. Such a jet subsequently interacts with its ambient gas, leading to a
reverse shock (RS) and a forward shock (FS). The magnetic field in the narrow
sub-jet is very likely to be mixed by an ordered component and a random
component during the afterglow phase. In this paper, we calculate light curves
and polarization evolution of optical afterglows with this mixed magnetic field
in the RS region of the narrow sub-jet in a two-component jet model. The
resultant light curve has two peaks: an early peak arises from the narrow
sub-jet and a late-time rebrightening is due to the wider sub-jet. We find the
polarization degree (PD) evolution under such a mixed magnetic field confined
in the shock plane is very similar to that under the purely ordered magnetic
field condition. The two-dimensional `mixed' magnetic fields confined in the
shock plane are essentially the ordered magnetic fields only with different
configurations. The position angle (PA) of the two-component jet can change
gradually or abruptly by $90^\circ$. In particular, an abrupt $90^\circ$ change
of the PA occurs when the PD changes from its decline phase to rise phase.
