Why would you ever want to track analytics with two services? We have many clients that have historical data in one service, or internal sites that they can't track with Google analytics. There are many reasons – but is it possible? Yes, with some caveats.
We have implemented some complex tracking and profiles – where not only are page views and click streams tracked – but as a video plays percentage based markers are fired off as events to Google analytics and Webtrends simultaneously.
Overall we have very few issues, but when they do appear during testing it can mean some elements need to be rewritten.