  We examine the drag experienced by a pair of vertical rods moving in tandem
through a granular bed immersed in a fluid as a function of their separation
distance and speed. As in Newtonian fluids, the net drag experienced by the
rods initially increases with distance from the value for a single rod before
plateauing to twice the value. However, the drag acting on the two rods is
remarkably different, with the leading rod experiencing roughly similar drag
compared to a solitary rod, while the following rod experiences far less drag.
The anomalous relationship of drag and the distance between the leading and
following body is observed in both dry granular beds and while immersed in
viscous Newtonian fluids across the quasi-static and the rate-dependent
regimes. Through refractive index matching, we visualize the sediment flow past
the two rods and show that a stagnant region develops in their reference frame
between the rods for small separations. Thus, the following rod is increasingly
shielded from the granular flow with decreasing separation distance, leading to
a lower net drag. Care should be exercised in applying resistive force theory
to multi-component objects moving in granular sediments based on our result
that drag is not additive at short separation distances.
