it might make more sense also to think of them as reduction cases rather than transfer cases. shamb did have a good explanation in that you have:

engine>transmission>(reduction case)>reduction case w/transfer>front/rear drive shafts>axles

the transfer case is what actually disperses power between the front and rear axles. so what people really mean when they say they have two t cases, is they have two reduction cases. the rearmost reduction case also happens to be a transfer case.

maybe that helps, maybe i made it worse