kind of a confusing question.
what size tank are you talking about? are you asking if 2 55w lights side by side on a 48" tank is the same as both lights only on one side of the tank??
if so, yeah, big difference. In the first scenario, you only have 55w over each half of the tank.
if you have both fixtures over one side of the tank, that 2 foot section is getting twice the lighting as scenario one.
Former advisor and planted tank geek...life's moved on though.