Not Send in the Cats! LOL!!! I would love this collaboration. they are two of my favorite composers, I'm sure they could come up with something amazing.
"Isn't there a YouTube of the two of them doing something together for a Cameron Mackintosh tribute?"
I've heard what they did for the Cameron Mackintosh tribute a couple of times and it makes for awkward and uncomfortable listening.
"The last time Sondheim wrote "just the lyrics" with anyone for a Broadway show was "Do I Hear a Waltz?" in 1965"
He also wrote lyrics to an unproduced musical version of The Exception and the Rule with Leonard Bernstein in the late 1960s. I've heard a couple of extracts from a demo recording.