Pierre Viret

Pierre Viret was one of Calvin and Farel's co-Reformers in the city of Geneva.  Though relatively unknown today, much of his work consists in what might be best described as popularized expositions of the theology of Geneva, as represented in their various confessions and catechisms, and more fully exposited in the work of John Calvin.  Most of Viret's work is in the form of a dialogue, and attempts to deal with common questions and concerns evident among the common people of the city. 
 
While he does not deal with the issue of the unity of the old and new covenants at any length, he does touch on it in one of his dialogues concerning the Roman doctrine of the limbus patrum.  This quote comes from his The Christian Disputations (1579):

            Theophilus.  I hope that we shall be soon agreed, and that there shall be in the end no other difference between the Saints that are dead under the old and new Testament, than between the living, who have been both in the one and in the other.  You cannot deny me, but that the Church of Jesus Christ had begun, whence the creation of the world, and the first that ever was just and righteous and which shall so continue unto the end and [illegible] of the world, and shall continue eternally.

            Eusebius.  I do confess no less.

            Theophilus.  It follows by the same reason that the Church of the Patriarchs and Prophets, which have been before the coming of the Lord Jesus Christ is none other but the very same of his Apostles, Evangelistss, and Disciples: and that the faithful of the people of Israel and those of the Christian people are but one people and one church.  For they had one very God: one very Christ: and the very promises: one very spirit of faith: and have eatech of one manner of spiritual meat, and did all drink of one manner of spiritual drink with us, as the apostle witnesses.  [pg. 245]

            Eusebius.  Then there shall be no difference between the ancient fathers and us.  To what end then served the coming of Jesus Christ?

            Theophilus.  I have not yet said so.  For I do find great difference. 

            Eusebius.  What?

            Theophilus.  As great as is between a star and the Sun, from the daybreak and the dawning to high moon: between the shadow and the body: between the Image and the truth: between the thing promised, and the thing given.

            Eusebius.  You will yet come to my matter.  For it seems by those words that they leave to the ancients but the shadows and figures, and that you give unto us the body and the thing figured: there is the great difference.

            Theophilus.  You do not understand me very well.  I do not deny, but that they had all that which we have: but not so clearly and so excellently and so gloriously, nor in such abundance of glory and majesty.  For by the coming of Jesus Christ the knowledge of God, and the intelligence of the holy Scriptures has been more greater, and more ample than before: not only in the land of Canaan, and to the people of Israel, but in all lands, countries and nations: and the spirit of God with all his gifts, graces and riches, has been distributed in more greater abundance, efficacy, virtue and magnificence than ever it was.

            Eusebius.  That which you speak of touches not yet but to the living, which have lived as well under the old as they new testament.  But what difference do you put between the living and the dead, under the one and the other?...