This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Bot can co-exist, you can even integrate Spring with your Struts Action. To do that you have your Action extend ActionSupport from Spring, which extends Struts Action class, but also has the nice ability to access the Application Context that you load when deploying your war file.
For both co-existing, is that the URLs that are already using Struts, keep them using Struts, but for new URLs, map tham in Spring to Spring MVC Controller classes instead of Struts Action classes.