A search for elements containing any of the other search data

I have a situation of having 2 databases:

Database A - has items that are things connected to orders, and each of the fields can have one or multiple order nrs associated with them.
Database B - has orders - from which I am looking for order numbers specified within a week (I have this solved out).

How should I combine those searches as I would like to look for Database A items which have any of the order nrs from Database B. And I never know how many order nrs were within a week.