Hi,
Have a look at this:
http://www.bicyclegermany.comThere is a "Heidelberg-Schwarzwald-Bodensee Radweg" which could be a connection between the two rivers.
Sorry, could not find any english information about it.
But this page might still be of help:
http://www.aufreisen.de/BWRad_WegKarte_0.asp?RIR=260518#hoehenprofilbtw.: there is also a thread in the german forum about the Rheinradweg.
Greetings
Olenizole