Have tested this and it is as I thought (in a CDX Cronus Database) <>xxxx&<>yyyy is the correct option.
I did have some issues though when building the filter, it doesn't quite seem to validate properly. I ended up removing the value from "Field Filter", closing page then entering entire value

If you want to exclude both then the first of those 2 options is the one I'd try. Did it work for you?

You can use the normal logic operators <> | etc but not wildcard characters.

Oops! Good spot, thanks for posting the link. Still can't edit here so we may have to repost.
Exhausting and exhilarating!