I cannot think in an easy way, but what I would do is offset 20ft or so one of the lines so they don't overlap when you merge them, import/merge both DON files, and then using swap "translate" the cables structures by structure from the offset line to the other. Of course you would need to be sure first that both models have the same feature code, criteria and alignment.
Depending on how long is your line and how many sections you have, it might be faster to simply string the second circuit from the scratch.