  Many debris discs reveal a two-component structure, with an outer Kuiper-belt
analogue and a warm inner component whose origin is still a matter of debate.
One possibility is that warm emission stems from an "asteroid belt" closer in
to the star. We consider a scenario in which a set of giant planets is formed
in an initially extended planetesimal disc. These planets carve a broad gap
around their orbits, splitting up the disc into the outer and the inner belts.
After the gas dispersal, both belts undergo collisional evolution in a
steady-state regime. This scenario is explored with detailed collisional
simulations involving realistic physics to describe a long-term collisional
depletion of the two-component disc. We find that the inner disc may be able to
retain larger amounts of material at older ages than thought before on the
basis of simplified analytic models. We show that the proposed scenario is
consistent with a suite of thermal emission and scattered light observational
data for a bright two-temperature debris disc around a nearby solar-type star
q$^1$ Eridani. This implies a Solar System-like architecture of the system,
with an outer massive "Kuiper belt", an inner "asteroid belt", and a few
Neptune- to Jupiter-mass planets in between.
