Select the track containing your secondary instrument and follow the procedure shown in Mixing Sense 5: Brightening a Dull Instrument. Your track setup will now be similar to that shown in the first illustration. In this example, the original guitar track is panned about 40% left and the copy about 40% right. We'll assume that you've done the same.
Now for the more featured instrument, our banjo.
Pan it dead center. Open the routing window for this track and create two Post-Fader/Post Pan sends, one to each of your guitar tracks. In each case, start with a level of about -5dB and pan the send the opposite side to the track to which it is being sent. Thus, the send to the original guitar track will in this example be panned 100% right and the send to the copy track will be 100% left. This is shown in our second illustration.
Play the song. You'll notice how much fuller it sounds, without loss of clarity or separation. As you play it tweak any of your settings to get the best results.