why do you need the propanolol with the valium, i am thinking that you need them for anxiety and/or panic attacks? both work quite well at treating both of those things, but propanolol is not addicting compared to valium.
i'd say go with 5-10mgs propanolol + your usually valium dose, don't really know why you would need the propanolol though....
They should be fine to take together in terms of safety although I imagine you could potentially feel a little woozy/dizzy/slowed down, so I would start low with the propranolol as laCster says and check how the combination feels to you. I am also wondering why you are needing to take both at once? Are you prescribed them both and have you used them before for anxiety (if that is what they are for)? Is 40mgs your usual diazepam dose?
I've been on a beta-blocker and clonazepam for years (decades, I think), and have never had a problem. My doctor keeps a close eye on my blood pressure, tho. I don't think one affects the other in terms of getting high, if that's your goal.