Hm, I'm not sure that you will be able to do this with InfoPath's SQL abilities - they seem to be a bit limited. However, take this advice with a grain of salt as I am not even close to being a SQL expert, and can't honestly say that I know what an "implied Inner Join" is.
What I would do is make two seperate data connections to both tables and then do the joining via XPath/XSL in the form. Whenever you need info from the other table, you can query it with XPath/XSL.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge