My general dictionaries give ´those involved´ (Concise Oxford) and ´united´ (Longman) for conjoint.
Oxford Advanced Learner's has
combining all or both the people or things involved
https://www.oxfordlearnersdictionaries.com/definition/englis...In your context, I would understand it as the (still married) couple, i.e. the spouses.
It is not, as far as I can see, a usual legal term - it does not appear in any of my dictionaries of law. However, I immediately think of the adjective conjugal, and my school Latin tells me the noun is conjunction, which does not fit, or conjoint, which does exist, although it is not very commonly used.