Better forward visibility for student and instructor.I like that Geoff, would you know why they chose the side-by-side arrangement instead.
It would make the nose-to-tail area distribution closer to the optimum Sears-Haack shape for minimum wave drag. Several contemporary aircraft, eg F-102, had lumps and bumps added for that purpose, process known as area-ruling.Apparently the side by side version was faster than equivalent single seaters, something to do with the airflow around the wider forward fuselage reducing drag. I wish I could remember where I read that.