2L and double battery!

Dragan, in very simple terms, when you engage 4H from 2H, you engage a transfer case that makes the front wheels tailshaft spin therefor giving you 4WD. Another issue, but you really only have 1 wheel driving at front and 1 wheel driving at rear unless you have diff locks fitted. (If 1 front wheel and 1 rear wheel loses traction, you go no where)
By engaging 4L, (Low Range as opposed to High Range) you engage a much lower set of gears which will give you more torque at a much lower speed. Better for controlling speed going up or down steep hills and other times. I've simplified it greatly but if you were to join a 4WD club and undertake training, you will find out a lot more about your vehicle and its capabilities.Dare to Lead not to Follow

Dragan,
Des has said the same as I would about the transfer case; and as the President of a 4 wheel drive club myself, I would be negligent if I didn't reinforce his advice about joining a club. You'll get a broad range of ideas and driving styles which you can choose from. Some blokes are way over the top and go at everything like a bull at a gate; tends to scare new-comers off. Just shy away from that type (if there are any in the club you look at) and learn lots of tricks from us loder blokes who've got nothing to prove to anybody.
Anyway, about the dual battery system. If your vehicle is already fitted with some sort of dual batteries, you need to find out how they are wired in. In a very few cases (eg: some Landcruiser 100 series I think), there are 2 batteries as standard, but they are "permanently" linked and aren't available to run accessories etc. Most of us who install dual batteries, do so in order to be able to operate fridges, 2 way radios and camp lights (as well as a host of other uses). In these cases we generally have some type of isolation/switching system so that we run such appliances only off our 2nd battery; ensuring there is always enough power in our main battery to start the truck next morning. Of course things like headlights, interior lights etc, if left on over night, still run off the main battery and have the potential to stuff everything up by flattening the main battery too.
In practice, most people fit a large enough 2nd battery to cover their needs and in the unlikely event of the headlights being left on all night, there MAY be enough power left in the 2nd battery to start the vehicle anyway.
Once again, if you join a club, you'll be amazed at the myriad of ideas that different members have. Nobody is right or wrong in their own ideas....it's like this forum; everybody simply puts forward their own experiences and you can pick and choose what's best for you.
All the best and welcome to the great hobby of 4 wheel driving!!
Cheers,
Roachie