This is already possible:
Can I use multiple tags for my question?
Yes! Just use the tag: command followed by a list of tags, separated by commas (e.g. tag: restaurants, relationships, Brooklyn).
You can also enter the tags as separate messages (e.g. tag: restaurants, then a new message with tag: relationships, then a new message with tag: Brooklyn). Aardvark will look at all of the tags and use them to figure out the best person to answer the question.
http://vark.com/faq