with MS it doesn't matter which injector you connect together since they all fire at the same time. the phrase "semi sequential" is a misnomer since they are just batch fired and the fact that there are two injector firing circuits is simply for load sharing
Correction: they CAN fire all at the same time, they can also ALTERNATE. Sometimes you have to run them alternating, depending on the size of the injector. I tend to run anything I do as two squirts + alternating. That then gives you similar duty cycles as if you were running fully sequential.
The two circuits are not for load sharing, although that is a byproduct. The purpose is for the above functionality.