  Recent observational and theoretical studies have raised the possibility that
the collimated outflows in gamma-ray burst (GRB) sources have two distinct
components: a narrow (opening half-angle $\theta_{\rm n}$), highly relativistic
(initial Lorentz factor $\eta_\rmn \gtrsim 10^2$) outflow, from which the
$\gamma$-ray emission originates, and a wider ($\theta_{\rm w} \lesssim 3
\theta_{\rm n}$), moderately relativistic ($\eta_{\rm w}\sim 10$) surrounding
flow. Using a simple synchrotron emission model, we calculate the R-band
afterglow lightcurves expected in this scenario and derive algebraic
expressions for the flux ratios of the emission from the two jet components at
the main transition times in the lightcurve. We apply this model to GRB
sources, for explaining the structure of afterglows and source energetics, as
well as to X-ray flash sources, which we interpret as GRB jets viewed at an
angle $\theta_{\rm obs} > \theta_{\rm n}$. Finally, we argue that a
neutron-rich hydromagnetic outflow may naturally give rise to repeated
brightening episodes in the afterglow lightcurve as observed in GRB 021004 and
GRB 030329.
