A largely uncharacterized process involving the release of duplicated spindle pole bodies (SPBs) and their migration away from each other within the nuclear membrane. Duplicated SPBs are connected by a bridge structure that may be severed in order to release the SPBs from one another. Following liberation, SPBs diffuse through the nuclear membrane until they are across from each other. SPB separation must take place in order for a bipolar mitotic spindle to assemble.