I don't believe this is possible without some kind of macro to copy one of the items and create the other (or take data from the user and create 2 items). Outlook stores tasks and calendar items in two different areas so even if you were to create both, changing one wouldn't change the other unless you had some kind of complicated add-in to manage them (AFAIK).
What are you trying to achieve by having a calendar-task combo?
I just want to chime in here for a bit of thanks. Your post pointed me to TaskLine and I have been blown away by it. I messed with Outlook for over 3 hours getting everything straight with the GTD Plugin and Taskline and the effort has been well worth it.
TaskLine created a schedule for me perfectly and I'm already amazed at the results. Using it is MUCH easier than the manual dragging-and-dropping of Tasks to the Calendar that I'm used to. Literally amazing!