Use google, find some offroad forums (try ZukiWorld
), do some research and find out what you're getting yourself into, and what your options are.
The purpose of an isolator is to allow both batteries to be charged from the same charge source (alternator) whilst keeping them separate (or isolated) so the load on one does not affect the other - this is how you would set your batteries up if you wanted to use one as a "camp site" power source, for lighting, maybe run a 12v fridge, without draining the starting battery.
The marine switch will connect between the vehicle's electrical system and the two batteries - it will allow you to select #1, #2, both or none - select either battery by itself and the battery that's not in use will sit there, self discharge & eventually sulphate (unless you remember to periodically switch from one to the other - lead acid batteries do not do well when left idle, they must be charged at intervals) - the people I know who use that style of switch end up leaving it in the 1+2 position - which is the same as hardwiring the batteries together.
Both battery negatives are going to be wired to the frame/chassis - what you do with the positives will depend on how you choose to wire it - if you use an isolator, wire as per the isolator manufacturers instructions - there are different types of isolator, so pay close attention to those instructions - if you use the marine switch, wire as per the marine switch manufacturers instructions, and NEVER turn the switch to OFF with the engine running.