You mean like a point of sale thing? Or like Amazon does where it says "People who bought this item also bought...."?
You could do a lookup table inbetween two items where you manually add the two products and then query that table.
Alternatively you could create additional categories for items that can be associated automatically with past items, so the category "Items that require AA batteries" could then be associated with 4 pack of AA batteries, and also Yapping Puppy Dog.
Of course you'd want to make sure there was the correct funnel for it so that if someone bought the Yapping Dog, they got the batteries recommended, but not so much the other way round.
The first solution is more work but may yield more useful suggestions for your customers thus resulting in more sales.