OK, I am going through beyond pure hell coming off of a benzo... Xanax currently. So I am going to be biased but honest. If you are on a beta blocker as it is you are treating pretty much the same symptoms as you would with the benzo so IMO it sounds like 2 drugs to treat the same thing. As what you describe the beta blocker doing is what some take in lieu of a benzo as it gives the same desired effect. I would bring that fact up with your doctor. Don't let them fill their "quota". Shrinks will throw as many drugs at you to supress every symptom you have and call you cured. When they stop working you have withdrawals and them dumping a new load of dope in your system to become addicted to.
Do you see a therapist beyond your shrink to deal with this?
Personally if the beta blocker is working and not addictive run, run as fast as you can from the benzo which is most certainly addictive. Because once additced you have the fun of withdrawals with PTSD and they don't go well together.
If you are taking the benzo to not get too stressed out then you are taking it for the wrong reason. It gets you pulled out of an attack which consist of the symptoms the beta blocker controls so I see no understandable reason any one would put you on both.
There is an article around here somewhere about this. I will try to find it.
How long have you been taking the benzo? Which one is it? If you were just started I would think long and hard on it as you already have the beta blocker.
Propranalol Click this to see info on the use of beta blockers