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I think your best bet is to use a parameter to filter the data sources. You'll find information on this in the following article: Filtering Across Multiple Data Sources Using a Parameter | Tableau Software
Your situation is a little different thought. Instead of just text (the field State in the article), you're filtering on dates. Hence, your calculated field for filtering would look more like:
[MyDate] >= [Parameter From Date] and [MyDate] <= [Parameter To Date]
I.e., you would have two parameters, and would create a field like this for each source, then filter using this.
Also, be aware that as of Tableau 10, filtering across data sources is supported by default! No need to use this "trick" then.
Let us know if this is helpful.
The other way to do this for date before Tableau 10 is to have one more Datasources with only dates in it. You can filter that data source and blend it with any other data source. It gives similar results to the parameter but you can use the native data filters.
You can populate the new Datasource with with a union of the dates in the other data sources if they are SQL.
As David Walker mentioned, beginning in 10.0, you can apply filters across multiple data sources. Here is the link again. As requested, you can filter across data sources in Tableau 10 | Tableau Software
If you're still having difficulty with filtering or have additional questions, feel free to reach out.