This is not currently possible, but it is a common request that I hope we can address in a future release (but I cannot commit to a timeframe). However you can still make use of data blending to capture some of the dimensionality from one data source and preserve it as an ad-hoc group in the other. This feature has helped other customers on this forum. See the following KB article for an example of how to do this: http://kb.tableausoftware.com/articles/knowledgebase/how-create-primary-groups-secondary-source
While you cannot perform a data blend without the relationship field, you can get the results you are looking for with custom table calculations.
I call it blending at one level, but aggregating at another. See Re: Data blending basic for an example, and a link to another thread with more details.
If you have any questions, or would like to see this applied to your situation, please provide an example packaged workbook that represents your situation.