The ol' copy and paste (Apple-C, Apple-V) also works just fine. This works with *any* object in FCP -- great option for when you need to restore things from an older project file. That's why I never keep more than one older, alternate cut on my current project file; this helps keep the file size down and speeds up access.
If the titles are already locked, you could also export a QuickTime movie of them and import them into both projects. This allows you to look at them without ever having to render again, unless you make changes to the titles.