We do not currently have the functionality that you described however I may be able to provide a workaround, albeit not a perfect one.
By having an even number of users on both schedules, you would be able to maintain each rotation being staggered. Perhaps you could add a couple of users to your Secondary schedule so that the number of users on that schedule’s rotation matches the Primary’s? You can add users more than once into a schedule, although that may be unfair to those users being on-call more than their peers. You could also place a dummy user into the rotation as many times necessary to even out to rotations. You’d just have to be sure to place an override anytime that dummy user is going to be on-call in order to have a real user be on-call, who won’t be on-call at that time on the Primary schedule.
I would be glad to submit this feedback to our product team in the form of a feature request. If there is any additional feedback you’d like to provide along with this request, please let me know!