okay what is the difference betwean twin stick and dual casses?
Twin Sticks is a setup that allows you to engage low range and 4wd independently. On some t-cases it allows you to disconnect the rear drive line and run in front wheel drive. The advantage of this setup is if you just want to go slow but don't need 4wd you can. It can also help with maneuvering your vehicle if you have a front locker on tight trails since the front end will not lock up and cause the vehicle to push through corners.
Dual t-cases gives you two low range options, but the rear t-case you still have to be in 4wd to be in low range. This has the advantage of allowing your gear ratios to stack up. You can have two 2:1 t-cases and have 2:1 or 4:1. Or you can have a 2:1 and 4:1 t-cases and have three gearing options of 2:1, 4:1 or both engaged for 8:1.
You can also get real fancy and run both a dual t-case and dual sticks.