My husband's cousin and aunt works on the same unit. They were both secretaries. As far as I know, the management will not place you on the unit when either one of you will be under the supervision of the other. Example: head nurse and a staff nurse on the same unit. Rules varies from unit to unit or per hospital. Better ask human resources as well about it for more info.
At a hospital that we did clinicals at there were twin sisters that worked the same shift,different units tho,I never saw them on the same unit together. But they made sure the same days so that they could car pool with each other.