this is a hyperthetical situation
basically a subwoofer works by moving still air, so a sub mot powered would make the air move ment look like this
different sized subwoofers are designed to run at different hertz so therefore moving different frequencies of air,
so where as an 10 inch sub might look like this
a 12 like this
and a 15 like this
they all distort the air around them differently, so if you where to have a 10 and a 15 working together you might not get the optimum performane out of the sub because it is already trying to move moving air