You pose a good question? And one only God can answer. I believe that's why so many churches believe that once divorced, there should be no second marriage unless the first marriage ended due to scriptural reasons.
It's too bad there is no "hotline" to God on these issues.
The Bible does speak of the reprobate. Maybe one who so defies the scripture as to compromise the vows of the first marriage and was a saved/believer falls into that category. They have wove a web so complex that it can't be resolved?
And see, God is not the author of confusion. If they're remarried, leave it alone. Jesus, when speaking to the woman at the well who had been married SIX TIMES, didn't say, now go find that first man and get back with him. No. IT'S OVER. So leave it be, is what the Lord would say. :)
Because, as I said in another post, commiting adultery through a divorce is a ONE TIME EVENT, according to the original Greek. It happened, and it's done. It's not an ongoing sin to "repent" of. Rather, stay remarried to the new person, and be faithful in that marriage :)