Talk:Trigger/@comment-2601:249:1500:DF0:14F1:5973:A445:5108-20180625213504/@comment-27002267-20180626055557

"Senkū can and must be equipped on the same side of the slots as Kogetsu in one's Trigger set, hence it can can be activated even if two Triggers are already in use." This is stated in Kogetsu/Senkū page. So yes, Tachikawa can activate Senkū through both the main and the sub while wielding two Kogetsu. Also, it is mentioned that both the Gen'yō and Senkū can be used in conjunction with the Kogetsu Spear.

However, I am not sure if this is exclusive to only Kogetsu based optional triggers or if it applies to all optional triggers.