We've been doing a lot of plastic double-contained piping, where the outer jacket is applied in the field using split pipe. In this case, we use separate line numbers and simply ISO the two separately. The carrier (inside) pipe is spooled/fab'ed as normal, and the outer line is set to all erection components.

For the few times we've done inner/outer welded systems, where everything is shop fabbed, we've used a system similar to what shollinger did.

I know that Intergraph's other software supports actual jacketed piping, and that ISOGEN has specific SKEY data set up for it, but so far no support for it directly in CADWorx.