  In a recent Letter (EPL 105, 40006; arXiv:1309.7107), Liu and Hu presented a
model of toppling-coupled sandpiles, where they found that the avalanche
exponents for two toppling-coupled sandpiles are the same as those for a single
uncoupled sandpile. In this Comment we provide a proof of this observation for
the case when there is conservation of grains in the bulk.
