I went into my surgery thinking it was only going to be an aortic replacement. Multiple echoes had shown that the aortic valve was clearly stenotic..absolutely no question that it needed to be replaced. There was also some evidence of "minor" stenosis in the mitral valve, but multiple doctors and my surgeon said it didn't appear to be significant enough to merit replacement, and hadn't changed much in 3 years, if at all. During my prep meeting with my surgeon, he stated that he would inspect it from the back side through the aortic valve during the surgery, and even if there was some minor stenosis, his tendency would be to leave it alone.
A TEE done at the beginning of surgery changed the course dramatically, as it showed that my mitral valve was actually as bad off as my aortic valve. They also found out at that time that I was suffering from secondary Pulmonary Hypertension (which has since resolved).
I had chest radiation for Hodgkin's 22 years before my valve troubles came up. My *personal* advice would be to discuss with your doctors the possibility of having a TEE well in advance of your surgery, as that would have helped clarify things for me and correctly set my expectations. My group of doctors tend to favor doing the TEE when everyone is in la-la land for the surgery.
Joy, I suspect the decision in your case would all boil down to timing...how close you are to the expected lifetime of your mitral valve prosthesis when it comes time to address the aortic. The big question for you here is that you're fairly young, and to do them separately would mean a total of 3 operations for you (counting the one you've already had). I'd bet that they'd take care of it while they were in there ... but I'm not a doctor
I hope this is helpful....