Two practical notes. When the shajo is divided into two, the one designated dai-ichi is the one closest to the kamiza. The threshold is an imaginary line inside the actual shajo area, about half way, and far to the back. Because that puts the kamiza to your left-front, the entry is a bit different: enter right-foot first, then the rei/yu toward the kamiza, and then the first step is with the right foot, parallel to the honza, before making a left turn to walk toward the honza and then proceed as usual. All of this seems to be based on the principle that your first step should be with the foot farthest from the kamiza.
Meanwhile people using the second shajo enter as usual, except that, instead of making the initial rei/yu toward the physical kamiza, its done in the direction of an imaginary kamiza at the right-front corner of their half of the shajo. If it were a shinsa, that’s where the shinsa-in would be.
That said there is considerable variety in dojo construction and the important thing is the principles.