Awesome. Ricky, a DT reader and dad of two, just posted a link to his homebrewed double stroller, which he crafted from two of Wal-Mart.com's closeout Quinny Freestyle 4's. I don't know what the JPMA says about this, and it'll probably void your dealer warranty [but then again, Ricky's solution doesn't alter the original Quinny frames or seats in any way, so who knows?], but it's definitely worth checking out.
The functional objective here was the Quinny Freestyle 4XL Plus, which accommodates chairs and carseats in various combinations (but costs EUR 510 before shipping at babycare.nl). With two regular 4's, some hardware and a drill press, Ricky's kids now ride in sweet Quinny comfort for less than half the price.