I was just playing with thread. I've not seen any patterns doing this, I'm not sure if I've invented or rediscovered...
Start with two shuttles with two colors. Hold the shuttles together and use as one shuttle. Hold the two threads together as one thread, and make the first knot of a ring to anchor the threads together.
Continue to hold both shuttles together, but work only one loop of the ring at a time. Let the second loop just rest.
open A loop in front of B loop
Thread A: double knot
open B loop in front of A loop
Thread B: double knot
Adjust slack on the two shuttles separately, and try to even them up to the same slack when closing the ring. Pull each thread tight individually when closed. At first I thought this would be easier with both threads on one shuttle, but only if you had a two bobbin shuttle and could adjust slack separately.
This gets you alternating color knots, with the two threads twining on the back, typical double-knot bars on the front.
I also played with things like interlaced knots and longer switch patterns...
I just worked up a tiny little piece, I can make something larger and post picts if I'm not clear...
This double bobbin shuttle tatter uses both threads in the core, but only one thread in each ring:
Anyone know of patterns that use two different double stitch threads in the same ring?