You will need to export the second movie (the one you added the titles to), then reimport it to an Event.
With the titles project open, from iMovie's menu select Share > Export using QuickTime. In the panel that opens, give your export a name and location where to save it (say, Desktop). For Export, select Movie to QuickTime Movie. For Options, then Video - Settings, for Compression Type select Apple Intermediate Codec. If checked, uncheck "Prepare for Internet Streamimg". The Sound settings should be OK. Now click on Save.
Once the file has been generated and appears on the Desktop (or your save location), import it to an iMovie Event using the menu item File > Import > Movies. Now you can drag the movie from the Event to your original project in the normal manner.
Sounds long winded, but in practice it takes very little time! Apple Intermediate Codec (AIC) is iMovie's preferred editing format and will preserve the quality of your original clips when exported in this format.
Message was edited by: John Cogdell