Android - 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.

I don't know how to use them simultaneously, but yes, you can have 2 languages. Make sure the second language is installed under settings. Then under keyboard make sure you tick the setting an extra key should appear when using multiple languages. Then when you are typing, let's say in whatsapp or so, go to your menu and tick the box of the second (installed) language under settings and voila, an extra button appears next to the space bar and on the space bar you see which language you are using.