When you are using more shafts than four you are able to make your back ground or foundation structure something other than plain weave. Of course the number of shafts greatly increases. This is still a two tie unit weave. See how the background is not plain weave. It is 2/2 twill. Over two, under two.
As you can see by this draft, I am not using a single two-tie unit. I am using a double two-tie unit. 1-3-2-4 is one of the units, 1-5-2-6 is the other unit (or block).
DOUBLE TWO-TIE : In this example: my unit is still four warp threads Two different shafts ( one and two) are reserved for the two tie-downs and two different shafts for the pattern warps (shafts 3+4 and 5+6).