I've only flown my Super Bird once, and haven't finished the Icarus yet, but they're essentially the same rocket with slightly different fins.
On the one flight the Super Bird has had, the NC and payload section ejected fine and *that* chute deployed perfectly.
I used some extra elastic to tie the NC to the elastic loop that passes through the 2 holes in the cardboard disc at the bottom of the payload section just in case the NC separated from the payload section, but that turned out not to be necessary.
The chute on the main body section ejected fine, but it failed to open, and 2 of the shroud line anchors, complete with the adhesive reinforcing things, ripped off the parachute. The parachute itself was still wrapped nicely in the remaining shroud lines and attached to the shock cord. The only damage was a ding to the front end of the main BT, which hit the ground first.
Anyway... I would certainly consider linking the two parts of this rocket together with shock cord as others have described, but what I'll probably end up doing is flying it without any payload section at all. It's long enough as is, and even with the payload section deleted, it's still stable according to RockSim.