Yes that is one way of doing it
+ to + and - to -
There are a few problems with this method one is that the two batteries should be of the same type ( age, crancking amps, charge rate etc) the reason being that load maybe drawn out of one faster than the other causing in balances between the two batteries ( like a seasaw of charge and draw ) and thus resulting in a shorter life span of the two batteries. Some people run this system for years with out a noticeable problem. The biggest problem with this system is if one of the batteris fails, a plate colapses, crackes and leaks or dies from age it will distroy the second battery in the process.
If the reason that you are thinking of a second battery for running a fridge then you should look at installing a deep cycle battery and not a crancking battery as it will last longer for this type of purpose. If it is for running spotlights or a winch the a crancking battery is more what you are after.
There are some great products out there that ensure both batteries are look after during charge and discharge.
Here is a great discription of various ways of setting up dual batteries.
Hope this helps
Adam
