pulled pork?

I would think just more surface area for bark production.  It should be the same as doing 2 smaller roasts.  Don't know how much time you'll save, though.  What's the rush?

Still new myself, but cutting the butt in two would mean two smaller pieces of meat.  The less thickness heat has to pass through to cook means shorter cooking times.  I don't think half of the time would apply, but two thirds to three quarters may apply.

The best thing to do is use a thermometer and cook to temp for safety and take as long as neccessary to get there.

Just my 2 cents.