Upload & Sell: Off
I'm not sure how it works for date and keyword types, but I know for Collection Name you can specify multiple values for the same operator just by separating them with a space. One I use, for example, is:
Collection Name Does Not Contain = a e i o u y 0 1 2 3 4 5 6 7 8 9
Essentially this will return all images that are not in a collection (unless the collection is name rather obscurely). Away from my Lightroom at the moment, but from this I'd think:
Keywords Contains = mum dad
Would return any image with mum OR dad as a keyword. Not sure if that helps at all.