can i add 2 more subs to my box?

Hi,

I have 2 15 inch orion subs in 2 bandpass orion computer designed box

So the back of the box is sealed , subs in middle and the front has the long vent airhole.. (bandpass)

My question is... I want to add 2 more 15s ,Would i mess anything up buy cutting a hole in the back of the box, adding anthor sub in . then run the one i added in reverse and  other normal..so there pushing and pulling toghter... Ive been thinking about this for a while..  but not sure if it would kill my subs,, Or just not sound right.. Last thing i want to do is mess up my boxes..  But i would like to have 4 15s instead of 2 15s

I rember back in the day i just simply screwed subs right on top of my normal 2 subwoofers in a reg sub box. Ran one in reverse .. was alot more db then with just the 2. 

i need professinal advise.. Thanks!!!