Make a 2nd item mandatory while booking PO with 1st item.?

HI,

I have a requirement that whenever i raise a PO for item A, another item B should also be part of the PO.

Like, Item A PO cannot be raised without item B.

Can we achieve this? Can we set up a rule like this?

in the item definition ( relationships) there is a relationship type called "prerequisite", but even after defining that its not working.