Although Smartimage doesn't create a URL to share multiple collections at once, there are alternatives to do this.

Option 1: Put all images you wish to share in a public collection, then use custom section headers to keep groups of images separate.

Option 2: Create a custom portal, which allows recipients to view a select  number of collections you choose.

Option 3: Use a URL shortening and bundling service, like bitly, to bundle all Smartimage collection URLs you wish to share into one link.