In my series on church authority (see here and here, for example), I have argued that when denominations are separate from each other in the context of a presbyterian system of church government, they are rejecting each others' de jure legitimacy and authority as a church (though not their de facto existence and reality as a church). We therefore have an obligation to avoid abetting schism by joining in formal membership only with that denomination which has de jure authority and refraining from joining formally with those denominations that do not.
So what does one do when one unavoidably lives in an area in which there is no de jure church present, when one cannot be under the governing oversight of a proper de jure church? Here is one possibility that I think makes sense:
First of all, consider an analogy: We can distinguish between de jure doctors and mere de facto doctors. A merely de facto doctor would be some person who has learned some medical skills and is actually functioning in a doctor-role, though without a legal license to do so. A de jure doctor is a person who has a legal license to function in a doctor-role.
Now, normally, of course, a merely de facto doctor ought not to be functioning as a doctor. It is illegal for him to do so. And people should not go to such a doctor for medical care. However, imagine a situation where a person is sick and in need of medical care but there is no de jure doctor within reach. What should this person do? If, upon investigation, it seems that the local illegally-practicing de facto doctor is reasonably good at what he does, and the sickness is at such a level that the person really ought to receive medical care, I would say that he ought to go and seek medical care from the de facto doctor. In this non-ideal situation, it would be appropriate for the de facto doctor to practice, and it would be appropriate for the person in need of medical care to make use of his abilities. The de facto doctor still has an obligation to attain a proper medical license if he can, and he is in the wrong if he does not do so; but in the meantime, due to the need present that only he can meet, it is appropriate for him to use his gifts to meet that need.
I think this sort of reasoning can apply to de facto churches as well. Ideally, one ought not to join in formal membership with an illegitimate church. However, when it is impossible to be joined to a legitimate church, it is, I think, appropriate to be joined to a normally-illegitimate church. The session of that church, which would normally be illegitimate, yet finds itself in a situation where it is the only available body to fill a need. Local people need shepherding; they need the sacraments; they need formal church fellowship and discipline; they need the preaching of the Word; etc. In this situation, although the de facto session still has a moral obligation to seek proper licensing (by coming into full communion with the proper de jure body) and it is in the wrong if it does not do so, yet due to its position by God's providence of being able to fill a need that cannot reasonably be filled any other way, it is appropriate for the session to use its gifts to shepherd the local people, and it is appropriate for the local people to join with it in formal membership, granting a temporary, non-ideal authority to it, given the circumstances.
I think that similar reasoning can sometimes apply to other spheres of authority as well, such as to the civil magistrate (I have discussed this a little in this post).