I just want to make sure I'm clear on what you have currently. If I'm grasping correctly, you have doubled the lead vocal and have a single track for the harmony. I would suggest also doubling the harmony. Then, knowing nothing else about the song, my personal default steps would be the following:
- Put the two lead vocals in a folder
- Put the two harmony vocals in a separate folder
- Pan each of the individual tracks either 100% Left or 100% Right in their containing folders
- Change the width control on the lead vocal to something like 10%
- Change the width on the harmony vocals to something like 50%
- Play with the width on both of the folders until you have a feeling of the harmony vocals "wrapping" around the lead vocals while not maintaining a realistic sound from the lead vocals.
As I said, this is the way I'd do it. There are tons of other approaches, none always right and none always wrong. But it's something you may want to try. I'm sure others will chime in with their own methods.