The spacer tube is a clearance fit on the steering stem & sits between both inner races. It needs to be around 0.010" longer than the length between the top & bottom bearing seats in the head stock tube. The reason it is necessary is to enable the bearings to be clamped together, as deep groove ball bearings are not designed to take a axial load.
When fitting the bearings drive the bottom one in first & make sure it seats.