Search Queries (get var's) and Multiple Post Types

I have 4 ‘post types’

1) Posts
2) Pages
3) Songs
4) Albums

What I’m looking to do is generate a search query string (GET) that only shows results from the “Songs” and “Albums” post types.

Here is what I’ve tried so far without any luck (Posts and Pages still show up in the results)

?s=keyword&post_type=songs,albums
?s=keyword&post_type=songs&post_type=albums

I think I’m just missing the proper separator for multiple post types.

How do I create a search URL using GET variables that displays results only from the two custom post types?