a standard push pull will do it, it can be one either the volume or tone - just buy the appropriate push/pull
it will look like this - but you will use the other side of the switch for the other pickup - wired exactly the same

(should say how to wire a coil split on the diagram)
except with BKP colours it will be green/white to the middle lug of the switch, red to switch or pot and black to ground
the alternative is to have 2x push/pulls both wired exactly as above or wired series/parallel. this will let you get the mixed humbucker/single coil settings