On the 365 C series, the shut down procedure is now to set one engine on ground idle (30 sec) while the other remains at flight idle. After these 30 sec, you shut the idling engine off and you set the one that was on flight idle to ground idle for another 30 sec.
It is said that the vibration frequency with both engines on ground idle doesn't match with the tail boom metalic structure.
This procedure is the same on AS 365N but different on N2 where you can set both engines at ground idle (30 sec)....The tail boom is made of composites here.