Hi HungNtree I nearly always fish 2 flys, on smaller streams or shallow riffle water I use a dry fly tied on to the end oo my leader then a length of tippet tied to the bend of the dry and a nymph on the end of that. In big water like the Tongariro or when I just need to get deep I replace the dry with a heavy nymph and the rest of the set up is the same (this is called "truck and trailer style" in NZ). I rarely go more than 6ft between dry and nymph but the 2 nymph rig needs to be much shorter, around 12 inches or you will get some mighty tangles.
If I am fishing still water I will fish with droppers tied as Jeff says but in moving water I get less tangles with the traditional Kiwi "truck and trailer" style as above.
All the best.
Mike.