If the same doctor prescribed both these medications for you, then you can trust it's safe to take them together.......but you must read the labels! Sometimes "together" doesn't mean "at the same time." Some meds need to be taken with food and some on an empty stomach, some in the morning and some at night. If you have any questions about how to take these meds, call your doctor or pharmacist for the correct dosing schedule.
We cannot give you ANY advice as far as what dosage to take. This information must absolutely come from your doctor and no one else! Please do not alter your dose until you've spoken with your doctor.
Propanolol is a beta-blocker, and can be used to treat heart conditions and anxiety / panic. Diazepam is a benzodiazepene. The two are safe to take together. I actually take drugs for all three.