Since you have not found the marriage record you want in the church records in Löningen, and you say they might have been married in Cincinnati, there are two resources you might try searching if you have not yet down so:
1. The Cincinnati main library has two different German language newspapers that were both in publication, I think, at the time of this marriage. If you also know the date of death of these ancestors, you might search the papers for an obituary on each, which often contained a rich amount of information. Be sure to search for a few days past the date of death. Look of course also for the wedding.
2. You may have already checked the every-10-year census records for Hamilton County, OH [Cincinnati]. The data kept at each census varied, but I know one of them at least asked how long the person had resided in the U.S., and all had the place of birth of each child in the family, a clue to immigration date often.
The Census records of course are also in Chicago. The Cinci German language papers are, I believe, not just in Cinci but also are at the Ohio Historical Society offices in the museum at the state fairgrounds in Columbus, but I can't recall if that is on microfiche or if they have the collection in paper also.