Like you declare war to take one duchy and the other person, if they accept, attacks another duchy nearby in a joint-invasion. Your forces would be friendly to one another. Similar to how the England was attacked by a coalition that wanted to seize the 4 english kingdoms. https://en.m.wikipedia.org/wiki/Great_Heathen_Army
The Great Heathen Army, also known as the Viking Great Army, was a coalition of Scandinavian warriors who invaded England in AD 865. Since the late 8th century, the Vikings had been engaging in raids on centres of wealth, such as monasteries. The Great Heathen Army was much larger and aimed to conquer and occupy the four kingdoms of East Angli...