Hi Dyna! Do you have the option to upgrade to Tableau 10? If you do, you can use the cross data source join introduced in that version to solve this issue.
The Problem is I don't want to join them. I want an Union. Data source 2 is a continuation of data source 1.
Oh, sorry, I missed that part! Cross-database unions aren't in Tableau yet, but they're on the road map.
I think your best bet here is to combine the data before it gets into Tableau, if at all possible. For instance, if you had some kind of data analysis middleware like Alteryx, you could do this very easily. Or you could pull them into Excel using data connections if that's an option.
Another option (if you have it) is to create some kind of a master list of IDs somewhere. If you had such a list, and that list had at least the maximum granularity you would want to put in a view, then you could use that as the primary data source and use both of your existing data sources as secondary ones.