...a common Cigar Box Guitar question...
I want to play 3 string CBG in GDg tuning on my 4 string CBG, what should I do?
Apart from just putting 3 strings on a 4 string CBG,
you can extend the tuning based on the full Open G Tuning (D"GDg" bd)
Add the low D...DGDg / 5151
Add the b...GDgb / 1513
The DGDg tuning may be easier because the highest string (g), is still the first string that is used most in bottleneck slide playing.
On CD 5 (4 string CBG), I use Dgbd tuning. If you retune to Dgbe, this is the same 4 highest strings found in standard tuning.
Together, DGDg and Dgbd creates the full 6 string tuning.
A great way to perform duets would be to use two CBGs in these two different tunings. This way you will have no choice but to play totally different ideas, making the overall sound richer and also avoid stepping on each other's musical lines.
Here is a similar example. I am tuned Dgbd and Bill is tuned gbd http://youtu.be/hTc9zYd8-v4