I have one of Rips (Source Automotive), twin turbo kits on my truck. I am using the HTB2 62/14 as the top turbo, with the S400 on the bottom. This combo works awesome for me. Great spool up and moves a lot of air.
I was running a Cool Twist as a single. As far as air flow is concerned it I was told it flows less then a 66, but more then a 64. The 62/14 and S400 combo spools a lot better then it did, EGT's are down over 200*, and it made 55hp more on the same dyno, with no fueling changes. I went with the HTB2 instead of using the Cool Twist, because I can go from the 62 to the 66 without changing the hotpipe or anything else. The Cool Twist (also an S300), is longer then the HTB2's, and would not be a direct interchange.
I've had a chance to drive Rips truck with a variety of top turbos. I've driven it with both the Air Werks, and the SP66 as top turbos. The SP66 on top is laggy, but builds lots of boost, and the air temps were way down. The Air Werks on top, spools so far that it will make your head spin (specially with the Mach 6+'s

).
Give Rip a call, and he can fix you up with a combo that works well, and is upgradeable if the need arises.

Here's the link. .
Source Automotive
Paul