Adding a Second Sub to a 7.1 System

I have just bought a second B&W DB1 for my Classe / B&W 800 Series system.

When I se the levels using an SPL Meter, do I set the level of the second (new) sub the same as the other sub and the speakers (75db) ?

Or, should both subs be set lower (as I am using two) ?

Thanks Tom