Holding dual certification is becoming less common nowadays. You can search ASHA's website for more info, i believe in the Omnibus poll. Back when the masters was the entry-level for audiology, it was more practical to become dual-certified. In fact, one could get a masters in either SLP or Aud and then continue on to the PhD and in the process complete the required coursework and clinicals for the other discipline. Like I said, ASHA should have some statistics to answer your question. However, if I were to guess, you could probably make another 5 to 10 k on top of your base salary for being dual certified but that depends entirely on your work setting. Like I said, dual certification used to be more common but is lessening due to the transition to the AuD. Is it useful? I think it could be in some cases (APD, pediatric/educational setting, cochlear implants) but futile in others (vestibular, tinnitus, dysphagia). Another good combination would be to get your PhD in clinical psychology. If your interest is in Aphasia or Epilepsy, maybe neuropsych would be a good path. If fluency strikes a chord with you then maybe counseling psychology is the way to go. If the educational setting is your interest then school psychology could be a good option. Neuropsychs make about 90 k so that would be a big boost in income; however, school psych would not boost your income at all. Your interests and desires are the most important thing that influence your decision; however, it would be foolish not to consider the financial cost-to-benefit analysis.