I have not actually done a dual case, but I have looked in to it a bit...

As I understand it, your options are fairly limited with the parts you have.

R151F transmission followed by a fwd shift or top shift* 2.28:1 Crawl Box followed by the top shift 4.70:1 transfer case with twin stick.

The Crawl Box does not affect 4wd, it only provides an additonal low range set of gears (2.28:1L & 1:1H) so a Twin Stick wouldn't have any value/function on a Crawl Box.

To get stock low range in 2wd you would simply put the Crawl box in Low (2.28:1), the Transfer Case in High (1:1) using the left shifter and leave it in 2wd on the right shifter.  You will have 3 shifters that all function independently.

Put the 4.70 gears in the transfer case (rear) to reduce the stress.

* I think that you can use fwd or top shift for the Crawl Box, but I'm not positive about it.