That will not help you if your alternator is not good enough. Batteries are NOT a subsititute for an HO alternator! Alt supplies power, battery does not, the battery only stores it.
The battery will not solve the problem like MS has already said but it will cover it to some degree, so if you have dimming lights and all that then it will help it slightly.
i would only recommend this if the car you have is not a long term.
Do you have lights dimming or are you running the volts down? Battery will help for sure, but if your alternator just plain can't keep up then you will run 12v on the battery not ~13.5+ on the alternator. And you will have to give it breaks to charge back up, though not as many as you do with one battery if that is the case. If it is just pulsating the battery will help but not as much as an alternator.