Only one supervisor but two reference letters required
For the second letter find someone who knows you well enough and has a high enough opinion of your work that they can recommend you without qualification. They need to make a prediction about your future success. Someone recent is better but absent that, an older relationship should work, generally. And someone in academia is better than someone else.
But there is probably no reason to delay your application to do another project. At worst, you could apply in parallel with starting another project as a backup.
Note, however, that my perspective is US, not Germany.