There appears to be 2 variations of the Xpipe design.
One variant is made by slicing the sides of the pipes and joining them at the opening. The SW seems to be of this type. A second type utilizes a common center box with the inlets and outlets merged. Some of these appear to retain the individual pipe contour as they merge or x-over.
Here's an example w 4" center to center.
http://cgi.ebay.com/ws/eBayISAPI.dll...r#ht_500wt_689