I saw Warren do this too and because I had already merged the High Hammond parts into a stereo file, I dragged the left and right B3 panners opposite. It really does make a difference in the sound.
"Merge to Stereo file" ... when you are looking at the file view on the far right, getting ready to drag the tracks into your session, if you right click, you get a menu. One of the options is the merge. It will stick the two tracks together, left is the one on top.
Caution: in 4.1 this causes a crash. So, I reinstalled 4.0.
You can also 'split to mono' files on any stereo file now! That was new with 4.0.
For the B3, because there are three tracks, and I had done it this way on another song, I loaded up the three tracks, panned the left and right, and then bussed the three together. Turn the bus into a folder bus then you don't have to look at the individual tracks again, but you can do fun things with them. I did some EQ on the center track, eg. but then also treat them all like one instrument.
"Folder Bus" Way cool Studio One feature. Stop me if you've heard this...
(Actually David Mood did a video about something for Produce Like a Pro where he did this. Don't remember which one off hand.)
Make a bus. Give it an interesting name like 'B3 Bus' and change its color.
Make a folder track. Or, select your three B3 tracks and "pack folder"
On the folder track, under the Mute and Solo buttons, is a box that says "None" with an arrow. Click on that and you will see the bus you just created in a pop up list. Select it and voila! your folder is now a bus. If you haven't started with the 'packed' folder, drag and drop any other track you want into it and off you go!