  We present a 'two-fluid' implementation of dust in smoothed particle
hydrodynamics (SPH) in the test particle limit. The scheme is able to handle
both short and long stopping times and reproduces the short friction time
limit, which is not properly handled in other implementations. We apply novel
tests to verify its accuracy and limitations, including multi-dimensional tests
that have not been previously applied to the drag-coupled dust problem and
which are particularly relevant to self-gravitating protoplanetary discs. Our
tests demonstrate several key requirements for accurate simulations of gas-dust
mixtures. Firstly, in standard SPH particle jitter can degrade the dust
solution, even when the gas density is well reproduced. The use of integral
gradients, a Wendland kernel and a large number of neighbours can control this,
albeit at a greater computational cost. Secondly, when it is necessary to limit
the artificial viscosity we recommend using the Cullen & Dehnen (2010) switch,
since the alternative, using ${\alpha} \sim 0.1$, can generate a large velocity
noise up to ${{\sigma}_v} \lesssim 0.3 c_s$ in the dust particles. Thirdly, we
find that an accurate dust density estimate requires $>400$ neighbours, since,
unlike the gas, the dust particles do not feel regularization forces. This
density noise applies to all particle-based two-fluid implementations of dust,
irrespective of the hydro solver and could lead to numerically induced
fragmentation. Although our tests show accurate dusty gas simulations are
possible, care must be taken to minimize the contribution from numerical noise.
