Threading four shafts on your loom for double weave is different from threading a four shaft pattern, because in double weave two shafts weave the top layer of your cloth and two shafts weave the bottom.
The most immediate difference is with e.p.i., as only half the warp ends are used for each layer – i.e. if you threaded your loom at 12 ends per inch you’d have 6 ends per inch in each layer of cloth. In other words you use double the sett you normally would for that yarn.
On a multishaft loom you need to watch how you thread and watch which shafts are raised and which lowered so the two layers of cloth don’t get caught together.
On a rigid heddle loom (I think) sett is simpler for double weave. Why? Well using two rigid heddles to get 4 shafts means you automatically double your sett, so the fact you only use half the e.p.i. on each layer cancels out the initial doubling! In other words, you just use rigid heddles of the size you’d normally use for that yarn.
Using two rigid heddles also means you can clearly see your treadling – one rigid heddle will go up to weave the top layer and the other will go down to weave the bottom layer. The two pick-up sticks, that allow you to move your slot threads, are also easily seen to belong to the upper or lower layers as one is literally on top of the other at the back of the loom.
Threading is probably where the rigid heddle is more brain-bending / eye-straining, but after having done a few double weave threadings now, I’ve got a system that works for me…
Begin with four warp threads in each slot; two each for your upper and your lower layers (see the left hand diagram below). If you warp directly onto the loom, you’ll probably do this first step then, but if you use a warping board, simply see this as stage one of threading.
You then take one of the upper layer threads out and put it through the front hole to the right and take one of the lower layer threads out and put it through the back hole the left (shown on the right below).
As I continue threading, I treat each group of four threads as though they exist in isolation – ignoring all the other threads. In the end, it will look like this: