What about expecting them to suck up their pride, work with the other lead for a comprehensive plan that includes possible WONTFIX-es?

Strategy and leadership don’t come to exist on their own. It’s middle management that has the best operational and tactical view bear none. Use that to influence decision making instead of complaining. (Yes, this is a theme in my professional life. Our middle managers don’t know their own worth. Pretty please give me Plans about what you Want to Deliver. Those are so much better than general strategies.)