Maybe you already know it, but, do you know that there's a bigger size?
SAMLA Box - IKEA
With it you don't have to connect two, but yes, it would be slightly smaller.
I heard that some people use a tube to connect the two bins (making a hole on them to fit the tube), I heard that other people just cut the right side of one, and the left side of the other, and they connect them with zip ties. But I've never have done any of those methods so i really don't know if they would work well