Is なきにしもあらず~なきに a set phrase?


This sentence recently appeared in one of my N1 training books:

> 彼女が合格する可能性は **なきにしもあらず** だが、彼がパスする可能性は **なき** にひとしい。
>
> My translation: While she may have a slight chance of passing the exam, it
> is hopeless for him. (lit. his chances are the same as nothing)

Is [なきにしもあらず~なきに] a set phrase that often/always comes as a pair? Or can I use
「なきにしもあらず」 also without the following contrasting 「なきに」 and still sound
natural?




無きにしもあらず ("not zero", "cannot say never") and
[無きに等しい](https://thesaurus.weblio.jp/content/%E7%84%A1%E3%81%8D%E3%81%AB%E7%AD%89%E3%81%97%E3%81%84)
("virtually zero", "nearly no") are two different set phrases which are not
directly related to each other. You can safely use one of them without using
the other.




無きにしもあらず ("not zero", "cannot say never") and
[無きに等しい](https://thesaurus.weblio.jp/content/%E7%84%A1%E3%81%8D%E3%81%AB%E7%AD%89%E3%81%97%E3%81%84)
("virtually zero", "nearly no") are two different set phrases which are not
directly related to each other. You can safely use one of them without using
the other.

