Hmm. This is one where I have to disagree with you, Darbury. To me, consistency is paramount, even over the subtle additional shade of meaning you might get by carefully considering and ordering the ? and ! based on that.
My preference is to pick one and go with it. I don't actually care which one, but my fingers do type ?! more naturally than !?, so in the absence of any better reason, I'd probably go with that one.
But the best option might be the direction Rooke is going: avoid the issue entirely. Most VNs are filled with more punctuation than a balloon is air - they can benefit from a little puncture and subsequent depunctuation. Drop a bunch of bangs and replace them with periods (or with nothing, if they were previously part of an interrobang), and you can usually end up with something that's probably better overall anyway. Especially if, in doing so, you get to reduce all the ridiculous double-bangs down to single bangs.