I'm very curious to know how did this work out for you? Did you order the spinners and install them yet? I have one of the original 2 player x-arcade sticks in my mame cabinet and want to add the same two-player racing functionality.
Yes, I did order the spinners (a master and a slave), and I went with the black knobs with the dimples, as well as the fly-weights.
It took me a little while to decide where to put them. I needed a place that both worked space wise with the stuff that is in the controller as well as a place that works ergonomically. I ended up using double-sided tape to stick the buttons to the controller in the locations that had room for the spinner to get a feel for how well gaming would work. The best place turned out to be at the top edge of the controller, in line with the player 1 and player 2 buttons, and between buttons 2 and 3 of the button arrays for each player. The Tankstick has 2 buttons in the back. One for programming and another one to disable the x-axis of the trackball. I had to relocate the programming button further to the left because it would interfere with the left spinner otherwise. Now I have an extra hole in the back through which I bring out the USB cable for the spinners for now. Eventually I will install some sort of strain relief for it so that the cable can't be yanked out.
The spinners work really well. I'm glad I went with the TT2s, they're really worth the money. Arkanoid, Tempest, SuperSprint, etc, are really great to play. GoldenTee Golf works really great too, BTW. It's a lot easier on the wrist than constantly having to whack the trackball. I use the right spinner to shoot and the left one to select clubs.
(http://farm6.static.flickr.com/5169/5374907782_9fc662a8dd_z.jpg)