Select the mono vocal track you want to enhance.
Copy it to an empty track. If there are no more empty tracks, use one that doesn't have any instrumentation playing during the portion to which you're trying to add the vocal, such as a percussive instrument that only plays during an instrumental passage, for example.
Pan the first vocal track to the left and the second one you just copied into a new track to the right. This can be done using the "Pan" effect which doubles the vocal while spacing it evenly across the audio spectrum to give it a stereo-like sound.
Equalize or "EQ" the vocal tracks to boost different frequencies that exist in your vocal tracks and strengthen them. This can be done by adjusting the "Frequency" and "Gain" effects until you reach your desired sound.
Add an effect, such as reverb, to both vocal tracks. Reverb adds an echo to the vocals which in turn makes them sound louder and fuller.