The deep cycle are designed to provide power for things like boat trolling motors, Computer UPS systems, RV lighting and accessories, things that draw steady power. They are not designed for starting a car.
The only reason to run more than one battery is that you have a monster sound system and you plan to deafen yourself and everyone nearby. Not a good plan, you will regret it in years to come. When pretty little Sweet Thing whispers in your ear, you won't hear her...
Its for competition purposes. need extra batteries because I do not have the skill to fabricate another bracket to hold a second alternator. if anyone does please let me know I will pay
They will work just fine. I use 2 agm batteries in my car, both in the trunk. Lots of cars use agm batteries these days, stay away from deep cycle marine batteries the vibration will kill them quickly. Make sure to use large enough cables too get a good ground path. I like to ground each battery to a different point in the trunk, the point of connection will only carry as much current as the size of the immediate surrounding metal. I use the rear seat belt mounting bolts, they are attached to a heavy mount that will carry plenty of current.
Good luck!
Oh and ground the alternator directly to the chassis with a heavy gauge wire, as short as possible.