The game allows multiple saves. Just make a different save at each Chapter break. Of course you'll have to start over from the beginning now in order to do that, but you won't have to do it a third time, and now you'll know to take advantage of extra save slots in other Viper games from the start (though RSR is one of the only ones long enough to really make it necessary, except for the ones with a complex set of storyline branches like V16).