If your company has existing Struts 1 apps that are working fine, I wouldn't bother upgrading to Struts 2. For new development, I think Struts 2 should get a serious look as it expands on the same fundamentals as Struts 1 but at the same time incorporating many lessons learned from production deployments and features from other frameworks.
In addition to the obviously excellent Struts 2 in Action book, I recommend taking a look at the Struts 2 migration guides, particularly the tutorials by Ted Husted and Ian Roughley:
I would like to add an answer to your title question. Yes, they can coexist in the same project. The only thing is that the servlet mappings to the actions are going to be different, so that struts 1 will manage some requests, and struts 2 will manage others
Hi Brown, Currently we are using Struts 1.2 on a project and we have developed some custom ways to make AJAX calls. Now we are looking to upgrade to Struts 2 to take advantage of things like easier Ajax integration in Struts 2. But after seeing you reply regarding upgrades, i was curious to know whether it would be worthwhile to go ahead with the upgrade?