Japanese dictionaries give both orthographies as equivalent . The word is Japanese, and with an unknown etymology [ibid.], so both orthographies are examples of 当て字. (In fact, the original pronunciation was ぐわい, with the わい > あい change emerging by analogy with the word 合.)
The graphical word is unknown to the Chinese language in either form , not defined by Morohashi , hence a jukujikun-style borrowing of the existing Chinese word is impossible. This puts any idea about the shades of meaning being differentiated under question, as such cases are normally applied when several existing Chinese words get matched to a single Japanese.
Thus, we only have to establish which is the more prominent orthography. Statistically , 具合い and 具合 are equally prominent, but the other versions appear disappearingly rarely. According to , for each 87.1% of 工合 with furigana ぐあい there is 12.9% of 具合, which means that the spelling 工合 is much more prone to be furigana-ed and thus less ordinary.
To conclude: both forms are as equivalent as various 当て字 of the same word can be; however, 工合 is disappearingly rare as opposed to 具合, and it is the latter that should be defaulted to.
 Absent in 漢語大詞典.