You are correct that you will need to use the Data Blending functionality if you are publishing the data sources up to Tableau Server. Currently there isn't a way to join published data sources at the row level. One thing to keep in mind when blending is that the "join" is going to happen after the results are aggregated to the level of detail of your viz. This will be important for your end users that are connecting to the published data sources.
As for the ORA-01795 error message, in the past I've seen this occur when people are publishing live connections from Oracle up to Tableau Server. Does creating an extract of the table in Tableau Desktop, then publishing the extract to Tableau Server get around the error message?