I don't believe there is a delay with the "shazzam" key. You can use a shazzam + other button with special adapter for primary inputs but it is HIGHLY unrecommended. For as long as the "special" button is held down ALL other buttons are in shifted mode as well. It's best to just use them for coin input and start buttons (something rarely pressed so they don't interfere with gameplay by shifting your entire panel on you).
It would also help to program all buttons that are not being used with a shift code to have the same shifted/nonshifted code.
You also have to watch for someone moving the primary joystick left or right at the same time someone presses the shazzam or dual button, that will throw you into an alternate codeset entirely...