??? your post is rather difficult to understand. If you wish you can refer to this website that gives instructions for adding a second battery to your electrical system.
http://www.fridge-and-solar.net/dual_bat.htm
When you add a second battery to your vehicle like the OP has all you are doing is basically creating one big battery. Lets say the batteries were side by side would you still connect the second battery to the frame of the vehicle or would you connect the two terminals together with a copper wire? Steel is a poor conductor of electricity it has a very high electrical resistance and will cause voltage drop as a result of its resistance. This is why while your way may work it is not the best way to connect the two batteries together. You even said yourself that high end competition vehicles use copper wire to connect the negative leads on the batteries together sure it will cost more but if your trying to be cheap why add a second battery at all? If your going to do the job do it right the first time.
As for a ground loop being caused by connecting the battery negative terminals together from what i understand a ground loop is caused by a difference in potential between the two grounds on the batteries. If your sole ground in the rear battery is to the frame or chassis of your car your going to have a lot higher resistance on the rear battery than the front battery because again steel is a poor conductor of electricity and the current flowing from the rear battery is trying to make its way back to its source in the front of the vehicle. Having a copper wire that goes from the front of the vehicle to the rear while it will have resistance the difference in potential will not be that great.
