How did "join issue" mean ‘jointly submit a disputed matter to the decision of the court’?
Kindly see the embolded phrase below. Etymonline is written too abstrusely.
The words issue and exit are closely related etymologically. Both go back ultimately to the Latin verb exīre ‘go out’. Its past participle exitus became in Vulgar Latin exūtus, whose feminine form exūta was used as a noun meaning ‘going out, exit’. This passed into Old French as eissue, later issue, and thence into English. The original literal sense of the word still survives in English, particularly in relation to the outflow of liquid, but has been overtaken in frequency by various metaphorical extensions denoting a ‘giving out’ – such as the ‘issue’ of a book or magazine. The sense ‘point of discussion or consideration’ probably comes from a medieval legal expression join issue, which originally meant ‘jointly submit a disputed matter to the decision of the court’, and hence ‘argue about something’.
Word Origins (2005 2e) by John Ayto. p 292 Right column.