I would think that you still only get the same amount of time. They have to be done and out of the park by a certain time. If there are two shoots that monring (5am and 6am) then one has to be done before the other starts. So, you basically still only have an hour.
I would still check with your EM if you can definitely have the carriage for your MK photo shoot. I has asked my EM about this because of the problems with the grad night lighting during our MK shoot. I was hoping they would offer the coach behind the castle as an option/alternative to having the obstructed castle during our MK photo shoot. Unfortunately she said that it would still cost $2,500 in addition to the MK shoot.