Yes, the n-grams (26) and the few WWW hits seem to be false positives.

Top 10 N-grams Lookup for 別婚 (Frequency Order)
別婚姻	271	  
別婚姻件数	214	  
別婚姻件数の	53	  

This is part of strings like ("number of marriages separated by..."):
年次別婚姻件数
都道府県別婚姻件数

Got here by chance searching "offing".  I had to look up "in the offing". I would rewrite with something more widely understood. "in anticipation of divorce", "with divorce likely".  *except*, I'm not sure this is a word at all.

Is there kokugo support?  This seems like a very specific gloss for a barely-existent term.

The way 別 gets next to 婚 is in strings like in the refs, and like:
シチュエーション別婚活方法
職業別婚活ガイド
年代別の婚活 / 年代別婚活
趣味別の婚活/ 趣味別婚活 ("searching for a marriage partner based on your hobbies", I think)

For example: 「趣味別婚活」の成功率が高い理由
https://www.itmedia.co.jp/makoto/articles/1402/04/news011_3.html

There is a book titled 別婚夫 on amazon, with the tagline:
結婚の持つ脆さ。愛を確信した女と男の信念の強さ。互いに外に愛人を持つ夫と妻。さまざまに揺れ動く男と女の心と性を精緻に描き、結婚は、夫婦の絆とは何かを探った小説。

That doesn't seem to confirm this gloss...

Otherwise, a search for usage is basically fruitless. 

別居, separate homes, is "separation", 別居婚 is "commuter marriage".  別婚 seems just as likely to be short for that as anything else.  I don't see how we would verify a meaning. 

At least two of the (few) google book results that seem to have 別婚 are actually misinterpreted instances of 別嬪(べっぴん).  I found one in a  reference book, and another in a novel here:
https://books.google.co.jp/books?id=4fBfAQAAQBAJ&pg=PT1285&lpg=PT1285&dq="別婚です""&source=bl&ots=PI5gpdtapJ&sig=ACfU3U36_9wUpO8tcRGiypICT5vO9hjXHQ&hl=en&sa=X&ved=2ahUKEwjFppC6_5yBAxUrgVYBHRZzB7sQ6AF6BAgOEAM#v=onepage&q="別婚です""&f=false

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