Same power or not, different subs will react to their enclosure/environment/the material differently. Instead of having speakers work together and compliment each other, it will make the freq response more erratic. But then you have people like skydeaner who hear it get louder, so they assume its 'better'.
Cancellation is a mis-used term in thissituation most times, its leads to alot of confusion.
In closing, chose one sub brand/model or the other. There is no logical advantage in trying to run both.