  Granular material in a silo with two openings can display a `flow rate dip',
where a non-monotonic relationship between flow rate and orifice separation
occurs. In this paper we study continuum modelling of the silo with two
openings. We find that the $\mu(I)$ rheology can capture the flow rate dip if
physically relevant friction parameters are used. We also extend the model by
accounting for wall friction, dilatancy, and non-local effects. We find that
accounting for the wall friction using a Hele-Shaw model better replicates the
qualitative characteristics of the flow rate dip seen in experimental data,
while dilatancy and non-local effects have very little effect on the
qualitative characteristics of the mass flow rate dip. However, we find that
all three of these factors have a significant impact on the mass flow rate,
indicating that a continuum model which accurately predicts flow rate will need
to account for these effects.
