Excluding multiple authors from query_posts

I’m trying to filter out authors on specific pages. I have no problem stripping one author from the feed using:

query_posts('posts_per_page=5&orderby=date&order=DESC&author=-18');

I cannot figure out how to add another author to the list.

`query_posts(‘posts_per_page=3&orderby=date&order=DESC&author=-18,-20′);’

and

‘query_posts(‘posts_per_page=3&orderby=date&order=DESC&author=-18,20′);’

and

and ‘query_posts(‘posts_per_page=3&orderby=date&order=DESC&author=-18,author=-20′);’

all remove one but not the other. How can I get it to exclude multiple IDs?

Thanks