Coactively is a term that is sometimes used in legal or contractual situations to refer to situations where two parties have to work together to achieve something. However, when it comes to finding synonyms for this word, there aren't many direct alternatives. Some possible synonyms that could convey a similar meaning include cooperatively, collaboratively, jointly, or in conjunction. Other related terms that could be used might include mutually, reciprocally, or symbiotically. Ultimately, the best synonym for coactively will depend on the specific context in which the word is being used and the desired connotations or nuances that need to be conveyed.