You can use both at the same time.
The body lift does as the name implies..lifts the body.
The leveling kit is usually nothing more then poly spacers that go on top of the front coils. Probably .5" or .75". Do you have any more info on the leveling kit?
Regardless, you CAN use a suspension lift AND a body lift at the same time.

EDIT: was just thinking...Is there a reason you are wanting a body lift instead of a suspension lift? if you are considering buying both of those you may just want to look into a budget boost suspension lift. The body lift leaves your clearence the same while a suspension lift would get you more. just my 2 cents