The Doubler emulates "Artificial Double Tracking" as it was done with tape recorders in the sixties. It is typically used on vocal tracks to make the vocals sound a bit 'fatter'.
The Delay knob sets the average delay time. Low delay values result in a 'fatter' voice rather than two separate voices. High delay values result in two separate voices. The Speed and Depth knob control the modulation.
The Mix knob sets the dry/wet ratio (0% being dry only, 100% being wet only). Lower values result in a subtle thickening of the sound without noticeable doubling.
If you want to pan the two voices to a different position in the stereo image you can use a Stereo Effect. Load a Doubler in one channel and set its Mix control to 100%. You can then use the Stereo Effect's Pan controls.
The Doubler effect can also model sixties style phasing/flanging effects, since these were done with tape recorders in a similar setup as well. The following settings can be used as a starting point for this:
Speed=0.18 Hz, Depth=1.5 ms, Delay=2 ms, Mix=46%, Phase Invert=On.
The Phase Invert button inverts the phase of the delayed signal. This results in a different sound, particularly at lower delay settings.