For this cloth, two shafts are used to carry the tie-down warps (shafts one and two). For this reason it is called a two-tie unit weave. It is also called "single two-tie unit weave" because I are using only one shaft for the pattern.
Shafts 1 + 2 carry the tie-down threads
All the other shafts carry the pattern threads.
But since this is a unit weave and four threads make up that unit..... you use the same shafts for the pattern threads.
So your unit could be: 1-4-2-4, or 1-5-2-5, or 1-8-2-8 .....see what I am getting at?
Single using the same shafts for pattern....
Two tie....using tie-downs on two separate shafts. (In this case - shafts one and two.)
Or you can call it Summer and Winter if you want to.... but single two-tie is better, I think.