Odata is not a database like MS SQL Server, and thus does not have separate tables in the same OData connection. Therefore you cannot use a traditional join to join multiple Odata connections.
Instead you will need to use a cross-database join, which is a feature that was introduced in Tableau Desktop 10.0. Therefore, to join multiple Odata connections you will need to upgrade to at least Tableau Desktop 10.0 and then use a cross-database join.
Hope this answers your question