Daijirin has this expression with a comma.

https://github.com/JMdictProject/JMdictIssues/issues/122
Adding commas to readings to align with kanji forms.

@@ -8,0 +9 @@
+<ke_inf>&sK;</ke_inf>
@@ -11 +12 @@
-<reb>ふたりははんりょさんにんはなかまわれ</reb>
+<reb>ふたりははんりょ、さんにんはなかまわれ</reb>

Daijisen: https://kotobank.jp/word/二人は伴侶、三人は仲間割れ-618865

>> jmdict does not aim to collect every random calque of western idioms

True, but we are usually comfortable having entries based on ones in major kokugos.
The entry causes no damage; is not misleading, etc., etc. JMdict is not being improved by trying to delete these sorts of entries.

仲間割れ	40891	99.8%
二人は伴侶	0	0.0%
ふたりは伴侶	0	0.0%
人は伴侶	95	0.2%

Explanation of the English idiom to Japanese people. Doesn't use this phrasing(because it's not Japanese, and has no explanatory power for Japanese people):
https://www.shane.co.jp/column/detail/id=18928
Two is company, three is a crowd.（2人なら仲間、3人は群衆）
「2人でいると親密な関係を楽しめるのに、3人では仲間割れが起こってしまう」という意味です。

My quixotic adventure continues.  daijs protects vocabulary, I get that.  But jmdict does not aim to collect every random calque of western idioms...

Based on a google book search, this showed up in a kotowaza book in the 1970s and made it into daijs(From which Marcus added it in 2018). Almost certainly someone's literal translation of the English equivalent. It has gained no traction. It is present nowhere(save for a couple kotowaza dictionaries, and one privately published test-prep book, below).

"仲間割れ" is common (and has a jmdict entry), "３人(で)は仲間割れ" appears by chance(still rare). Explanations of the English idiom use something like "２人は良い連れ".  "get along well".  No one uses "伴侶" for this.

It should be deleted because it serves no purpose here.  It will never be encountered in the wild, if it is encountered, its meaning is self-evident, and most importantly, no English speaker should use it and expect to be understood.  Marking this "rare" just leaves a trap for people who look up "伴侶" and stumble into it. (or who look up "two is company"). 

By chance the one place it pops up is a Tokyo University question booklet that asks, "give two idioms with the same number and connection". 
Q26 「二人は伴侶三人は仲間割れ」ということわざでの数字と関連があることわざを 2 つ挙げよ。ヒント...
Here is that book:
私の東大クイズ
https://www.google.co.jp/books/edition/私の東大クイズ_3/ib4MAwAAQBAJ?hl=en&gbpv=1&dq="二人は伴侶、三人は仲間割れ"&pg=SL17-PA26&printsec=frontcover

Note, the book doesn't expect the student to know the idiom.  It's just used as fodder to prompt for an actual Japanese idiom.

The book is not published by the university.  It's got two reviews and 1 star on Amazon. Reviews say it's garbage.

https://www.amazon.co.jp/私の東大クイズ―悩みから生まれた305問-鍾-非/dp/4780706254

daij