Stanley's basses have one truss rod because the first ones he had only had one and he's very traditional. Notice his bass pickup is very close to the end of the fingerboard. Again, the first basses we made for him were done like this, and had no truss rod cover, so we still follow this tradition.
Another practical reason for a single truss rod in Stanley's basses is that his fingerboard dimensions are very narrow and the neck is carved very thin. A dual truss rod simply doesn't fit in this configuration.
I pulled the file for the picture you posted, Pierre-Yves, so this information is not from memory, but from the documentation.