Are your fact tables completely disjoint?
I'd suggest a third table or perhaps your date dim containing every day between the earliest and latest dates in both fact tables as the primary table.
Then join fact tables to it with outer joins.
You'll need to aggregate fact tables that have multiple rows for the same date to prevent duplication of the other fact table
Some dates will have both sides, Some will have nulls on one or the other. If you have more than one row per date