I have some order data that I want to combine with every possible combination of products, grouped by order ID. That way I can show the products that most frequently appear together on the same order. It looks something like this: OrderID ProductID 1 3 1 7 1 4 2 5 2 8 2 6 and for analysis I want to ...

Tableau measures are set to a SUM aggregation by default.  You can change the aggregation for a measure by 1) right-click on the measure 2) select default properties and 3) choose an aggregation.  Each time you drag the measure to the canvas it will maintain the select aggregation.