This week's book giveaways are in the Refactoring and Agile forums. We're giving away four copies each of Re-engineering Legacy Software and Docker in Action and have the authors on-line! See this thread and this one for details.
There's no reason they can't exist side-by-side in the same application. There is also a Struts 1 plugin for Struts 2 that allows the use of S1 action classes, but it doesn't let us use S1 tags, so I'm not sure it's worth it.