My favourite trick for double-tracking effect is to use a Chorus block between an Amp block and a Cab block in Wet/Dry/Wet configuration. This is actually from Brian May's current chorus settings, and personally I find it the most natural double-tracking effect.
Attached is a sample Preset with my double-tracking effect, so give it a try if interested (make sure your unit has Ares 2.0 firmware. You can try Scene2 and 3, too).
Here's the details:
Connect the blocks as the screenshot below, set the Chorus/Amp effect type in stereo mode, and hard-pan the Cab's two channels against each other. This creates Wet/Dry/Wet configuration.
Then, set the Chorus block as the screenshot below. "Phase reverse" is the key ingredient: reversing one side significantly widens up the sound. To get more width, I recommend LFO Phase to "0.0 deg," Spread to "100%", and High/Low-pass filters in Tone page not engaged. Finally, adjust Depth and Delay Time to your taste. In my preset, Delay Time is set to my "sweet-spot" where maximum width is obtained and low frequencies are not canceled out.
