While in some cases, doubling a vocal line (singing it in unison) can be a neat effect, it isn't always terribly effective in "fattening" the lead vocal sound. If anything, it can "thin out" the vocal. It can cause a sort of comb filter effect.
Frequency "coupling" is where frequencies are totally in phase in the time line. Frequency "cancellation" is where frequencies are 180 degree's out of phase in the time line. Of course, you will seldom ever have total phase cancellation of an overall sound because most sounds are a combination of several frequencies combined.
So, you double the vocal track. One some frequencies, you will have coupling. In others, you will have partial or total cancellation. The skill of the vocalist will have a lot to do with this.
What I have found works best in the vocal doubling technique is when the vocalist can create a different timbre with their voice. For instance, on one track, they sing with more resonance, and on the other track they singer with more syllabance.
This is complex. Sorry, I just don't have the time right now to explain the whole thing in more detail. I guess what I am trying to say is, vocal doubling (unison) may NOT "fatten" the vocals. It may in fact make the vocal part more "thin". It really depends.
Try it out and see if it works out for you.
Ed