In data models designed around business processes, we will often have just one source fact table. If we have additional fact tables, they tend to be at a similar grain to the main fact table, which is easier to deal with. Line-of-business documents may have fact tables from lots of different sources that are not at the same grain level at all, but we are still asked to deal with creating the associations. There are, of course, several methods to deal with this scenario.
If the fact tables have an identical grain, with the exact same set of primary keys, then it is valid to join, using a full outer join, the two tables together. Consider the following example:
Fact: Load * Inline [ Date, Store, Product, Sales Value 2014-01-01, 1, 1, 100 2014-01-01, 2, 1, 99 2014-01-01, 1, 2, 111 2014-01-01, 2, 2, 97 2014-01-02, 1, 1, 101 2014-01-02, 2, 1, 98 2014-01-02, 1, 2, 112 2014-01-02, 2, 2, 95 ]; Join (Fact) Load * Inline...