dual cases
Well there are two kinds of dual case setups or "doublers". Married and divorced. Married, means just that, two cases bolted together. Diveorced means they are mounted inline seperated with a stub or jack shaft between them.
You can also buy multispeed transfer cases meaning they have several gear ratios to choose from.
I have the dana 300 to NP205. Essentially this gives me a chep multispeed transfer case. Double kits cost around 600 plus the extra case. I twin sticked the 300 but only hooked up the high low stick as the drop is wrong side and left the front engagement out. If this were run to the front axle, you can engage front wheel drive and do front digs.
I use the front case (D300 as a gear splitter). It has a low ratio of 2.61 to 1. I can kick it in for 2LO, pull stumps, gear down for whatever may be needed. Then when I engage the 205, I have 4hi (1:1), 205 in lo (1.96:1), 205 hi, d300 low (2.61:1), d300 lo, 205 lo (5.12:1). There are four ration in 4wd I have.