1. The first general ground, which we take for granted, is this: that by way of precept there is an absolute necessity of uniting laid upon the church, so that it falls not under debate ‘Whether a church should continue divided or united in the Theses?’ more than it falls under debate whether there should be preaching, praying, keeping of the Sabbath, or any other commanded duty; seeing that union is both commanded as a duty, and commended, as eminently tending to the edification of the church, and therefore is so frequently joined with edification. Nor is it to be asked by a church, what is to be done for the church’s good in a divided way, thereby supposing a dispensation, as it were, to be given to division, and a forbearing of the use of means for the attaining thereof; or rather supposing a stating or fixing of division, and yet notwithstanding thereof, thinking to carry on edification. It is true, where union cannot be attained among orthodox ministers, that agree in all main things (for of such only we speak), ministers are to make the best use of the opportunities they have, and during that to seek the edification of the church. Yet, that men should by agreement state a division in the church, or dispense therewith and prefer the continuing of division, as fitter for edification than union, we suppose is altogether unwarrantable.
(1) Because that is not the Lord’s ordinance, and therefore cannot be gone about in faith, nor in it can the blessing be expected, which the Lord commands to those that are in unity (Ps. 133). (2) Because Christ’s church is but one body, and this were deliberately to alter the nature thereof. And although those who deny this truth may admit of division, yea, they cannot have union, that is proper church union, which is union in government, sacraments, and other ordinances, because union or communion in these results from this principle. Yet it is impossible for those that maintain that principle of the unity of the catholic visible church, to own a divided way of administrating government or other ordinances, but it will infer either that one party has no interest in the church, or that one church may be many, and so, that the unity thereof in its visible state is to no purpose. This then we take for granted. And though possibly it is not in all cases attainable, because the fault may be upon one side, who possibly will not act unitedly with others, yet is this still to be endeavored, and every opportunity to be taken hold of for promoting of the same.
Durham points out clearly that unity is an absolute duty. It is never warranted for the church to exist in a divided state. This is always a sin. So when two church bodies recognize each other as true parts of the church de jure, they are under an absolute moral obligation to exist in a united state. If the OPC and the FPCS, for example, recognize each other as legitimate churches, they both have an absolute moral obligation to do all they can to be united to each other at once. If one of them refuses to do so, it is committing the sin of schism by dividing the church of Christ.
If two denominations remain divided from each other, according to Durham, "it will infer either that one party has no interest in the church, or that one church may be many, and so, that the unity thereof in its visible state is to no purpose." That is, it will infer either that the two denominations don't recognize each other as being legitimate parts of the catholic church, or that the denominations have rejected the universal catholicity of the visible church (and thereby denied a fundamental principle of presbyterian church government for a form of independency).
For more, see here, here, and here.