Set two languages simultaneously for the stock keyboard?
The stock keyboard in Jelly Bean (4.1 or 4.2) does not support multiple languages simultaneously. However, SwiftKey, a third party keyboard, supports advanced predictions with up to three languages simultaneously.

SwiftKey Official Website
SwiftKey on Google Play

Note that SwiftKey is a paid app. You can find the non-trial versions of the application in the Play Store. If you are looking for Swype-like functionality, note that an upcoming version of SwiftKey also includes it.