there is no way/command to set a 'prefered active slot' for the supervisors.
the first sup initialized/online becomes the 'active supervisor engine'.
(no matter if in slot 1 or 2)
you can force a supervisor to become active with the 'reset module_number' command as described below.
if you have an active sup in slot2, then your slot1 sup is in standby mode. to get sup1 to be active, use the 'reset 2' command.
this will reset the sup2 in slot2 and initialize the sup1 in slot1 to become active.
please see the following link for more info:
http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_guide_chapter09186a008007fa3c.html#1019903
