Yes, you can. Here's how:
1. Go to your VST Connections window (F4) and click the Studio tab at the far right. (Click on image below to enlarge.)
[url=http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/files/mpv_41278_93591_1.png] [img]http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/thumbnails/mpv_41278_93591_1.png[/img] [/url]
2. You'll need to add at least one Cue. Since most people use Mix 1 for their studio monitors, I'd recommend Mix 2 or Mix 3 (as in the above figure.)
3. Then go to the Devices menu and select Audio Hardware Setup at the very top of the list, and make sure the Headphones tab is selected. (Click on image below to enlarge.)
[url=http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/files/mpv_41278_93591_2.png] [img]http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/thumbnails/mpv_41278_93591_2.png[/img] [/url]
4. Set PHONES2 to the same Mix as the Cue you created in step 2.
Now you'll need to go to the MixConsole and modify the Cue level and pan controls of each channel to customize the headphone mix.