The Spring 2018 patch introduced the ability to apply casus belli to joint wars. In my current game, I have denounced Philip over 5 turns ago, and I have the Formal War casus belli against him when declaring war on my own, as I would expect. However, Catherine has also denounced Philip, and I can offer her a joint war against Philip under the "surprise war" casus belli (with the attendant egregious warmonger penalty), but see no option in the diplomacy screen to offer a joint war under the "formal war" casus belli.
Can joint wars be declared using the "formal war" casus belli? If so, what conditions must be satisfied for this to be possible? I suppose it's possible that I lost track of when Catherine denounced Philip and that both of us must have denounced over 5 turns ago, and that she has not denounced him long enough, but that's the only explanation I can think of unless it's altogether impossible.
Edit: In case it matters, this is the vanilla game, without Rise and Fall.