It is possible to reuse an AU Graph exactly in the same way as described in the question.
The problem why it did not work for me was that I had a bug which caused interferences/overwritings between my two AUGraph instances. I found these bugs only by chance. Since I fixed them, the AUGraphs work as intended.
When keeping the two AUGraph instances completely separate, you can use and reuse as many AUGraph instances as you like. I did not test if they can be used simultaneously.