No Canonical Agreement without a Doctrinal Resolution First
It would seem that this is not a Catholic principle as such, but rather a proposed mode of action which is based upon a number of other Catholic principles.
And if the Rome with whom they treat is one which holds the required faith of the previous Magisterium, then they would be negotiating with with Catholic Church.
But if the Rome with whom they are bartering holds the conciliar doctrine in contravention of the prior Magisterium, then they would be haggling with the Conciliar sect, not the True Church.
So this proposed mode of acting is in fact based upon whether or not they are dealing with the Conciliar, or the Catholic Church.
As an aside if the entity that is in question holds the true Faith and is thus defined as the Catholic Church, then the Society and the resistance would not be agreeing or negotiating, they would be submitting.
Let's face it, doctrinal resolution means the conciliarists rejoining and converting to Catholicism. Nothing more, nothing less.
Conciliar or Catholic is a fundamental predicate to your proposed action.