I have two subwoofers, my question is could i put one in a bandpass box built for it to get the added dB's and the other in a sealed box for the broader hz range? Would this create any odd ball effects (assuming a correctly built bandpass box)? or would it be better to just leave them both in sealed boxes?

hmm could be tough to get them to sound right in two different boxes, but im sure its possible. i would probably recomend that you just put them in a sealed box, cause it will provide the best sound in our cars.

Yes you can do it, provided you have used a good computer box building program, and you know exactly what frequencies the bandpass box is letting out. You would need to know the cutoff point(like 60hz,80hz,ect..) to know where to set a crossover for the beginning point on the other box. You dont want to mix them, cause the bandpass and a ported box have slightly diffenrent dropp off slopes, and it will sound pretty bad IMO. You are just better off designing a good ported box if you really want some db's. Be sure the subs are built to handle a ported box, and if the box is designed properly with the correct port size and diameter, the box will be loud, and clean sounding. Its all in the design..Never do any guess work with ported boxes, and esp with bandpass boxes. They are very sensitve with the airspace and porting sizes..