Hoagie_one has the simplest solution, but I wouldn't pick the P1+P2 combo as it isn't the best choice.
As has been pointed out on numerous occasions, in the heat of 2-player simultaneous games, players can lose their last lives and both reach for the start buttons as the same time to continue. This would result in exiting the game.
Use a combo that only P1 is in charge of and is unlikely to happen by mistake. Perhaps, P1 Start and P1 button 6 and/or 7 (a 3 button combo is even more secure)
But if you really want to, you can pretty closely simulate the behavior you are used to;
Hook both the Shazaaam! and the P1 Start input to the P1 Start Button. Then upload a custom codeset to the KeyWiz with the "Shazaaam!" key for the P1 Start input set to "1" and the "Shazaaam!" key for the P2 Start set to "ESC".
The one drawback of using this method is that the "1" key will be sent before any of the shifted ones, but unless the game is sitting there waiting for you to start, this will be ignored. So the net result is the behavior you are used to with little to no side-effects.
RandyT