Hello Jikurru28
I am not sure whether it is even possible to cancel a Swiss Travel Pass online. If it is, there should be a link in the confirmation e-mail to do so (not a reply, a link which will take you to an SBB ‘ticket adjusting’ page).
If you bought the passes without login, there will not be a link. If that is the case or if it for other reasons does not work, the easiest way to have the situation cleared up is via phone (+41 848 44 66 88 at -.08/min).
As to my knowledge, it should be possible to cancel only one of the passes.