Since the original launch of NavWow (faceted search navigation), one of the biggest requests from customers has been the ability to select multiple search facets at the same time. In the next couple of weeks we'll be launching that multi-select ability in NavWow. It'll be exposed only to a randomly selected subset of customers and you might find yourself seeing it (if you followed the link from a search to this blog post, you're likely in that test! Congrats!). Lots of work has been put into this by our cross functional search team.
Here's Dylan Bathurst explaining NavWow:
We have more changes planned in the next few months but we think this is a great start. We're looking forward to your feedback!