It's a neat little mark but I don't think it has much of a chance being widespread because I think ?! has already won in real usage and doesn't have the fine detail problem. I think it has a better chance of becoming an official grammatical digraph than the merged mark.
I don't think it has much of a chance being widespread
I first heard of it back when Usenet was still the hotness.
Apparently it's been around for close to a century, so if it hasn't caught on widely by now, you're probably right.
It lost when computers overtook typewriters.
On a typewriter, you could type one punctuation mark, backspace, and then type another.
Even once computers added the interrobang to Unicode (or older systems), you either need to type in a numeric code, use a secret bucky bit, or select it from a graphical UI, none of which is worth it for more than the odd weirdo with a whimsical streak.
Is there an overlaid ?? and !! mark? (For Chess.)