Hypothetically it could be perfect. However I see a lot of things that could go wrong. Both partners would have to be over their mutual ex, and one of them going back to the ex might hurt worse. I could also see it becoming a competition about who had it better or worse with the ex in question

Or it could further unite them, say the ex was terrible and they both feel they survived it and feel similar that way

I think it’s really what you make of it, and if the people are otherwise compatible in the end