Here's a great tip from Sweetwater Sales Engineer Kenny Bergle for creating a double tracked effect.

It's common in the studio to try to thicken the sound of a part like a vocal or guitar riff by applying effects that simulate the sound of double tracking. Accomplishing this usually involves the use of a delay, chorus, or a Harmonizer (pitch shifting device), or often some combination of these. While each of these effects produces a unique and usable sound that can be very effective - and I use them often - there is always something electronic or synthesized sounding about them. Some manufacturers have even employed some pretty sophisticated randomization algorithms in their boxes, which can really help, but still comes up short for some tracks.

Most recently I've been using a pitch correction device to accomplish this and the results are amazing. I use Autotune, but there are other products (hardware and software) that can do similar things. All you do is record your part as you normally would. Then you apply pitch correction to the part, but rather than simply correcting the pitch of the track you bus the corrected part to another track, which you can then use as a double track. Depending upon how you set the parameters in the pitch correction and how you mix the two tracks this produces some amazing results. It tends to have a much more organic sound and feel compared to parts that are created by more static processes. And since you're pulling the pitch more towards the correct value, as opposed to away from it, as is the case with things like chorus and Harmonizers, you can be much more heavy handed in the mix without it sounding as "effected." Additionally I will sometimes add a small amount of delay, or a slightly modulated delay, in the signal path before the Autotune, which gives it a little more separation.

There will probably never be any electronic method to perfectly recreate the sound and feel of a double tracked part, but this method comes as close as anything I've ever heard.