These are stacked potentiometers, and
are useful for putting a volume and tone knob together to save space either
to give a more spartan appearance or to leave room for more controls.
They can be difficult to come by since most standard guitars do not use
them. However, they are available from Warmoth
among other places.
Note: While they superficially
blender potentiometers (aka dual-ganged
pots), the separate elements (i.e., top and bottom) may be operated independently
of one another on concentric pots.