Use millis() to time when the first button press was detected.
Use state detection: you want to react to the start of a button press (the signal going low), rather than the button still being pressed (the signal being low).
Also do look into debouncing your buttons as that's another source of problems.
You also have to decide on exactly how you want to have the buttons behave. Both press and release will never happen at the exact same time, so how long to wait before deciding it's just button A, not both? For this you also have to keep track of the state of the button: keep checking that a signal remains LOW for some time.
So now you have an A+B button press, of course one gets released first. What now? Do you want to react to the other button as single button press? I don't think so - that's where you have that state detection. You don't do anything new as long as the button remains low. Just wait for the button signal to become HIGH before doing anything else.