Currently you cannot use more than one zip code (or city, or tract) via shortcode snippets. In the meantime however, if you also have the dsSearchAgent Pro product, you can:

1. Login to your dsSearchAgent LINKS area and create a custom link that uses multiple zip codes as the search criteria (Read: How To Create Custom Links), then…
2. Use shortcode like this to show those listings:

[idx-listings linkid="123456" count="50"]

…but otherwise, shortcodes will only show one city/zip/tract.

Note: You CAN specify multiple values using the Custom Link Structure. For details on this, read:

How To Specify Multiple Values Using the dsIDXpress Link Structure