Dual 12" Sealed Enclosure Question
 

I just made a box 3.5 cuft for 2 12" subs that recommended a max size box of 1.75 cuft per sub. Do I have to put a divider in between the box to separate them or can I leave it open? Thanks.

No you do not need to add a divider, by utilizing two drivers you double your compliance volume, but all other parameters remain the same. Though the one benefit of adding a separator is that you can run with one sub. No divider and you'll be out of luck if you drop a driver.

Is there anything to gain by not adding a divider?

There are several arguments for not dividing the subs. First off, chances are that the subs are no exactly the same (manufacturing variation and all that) so if you separate them, you can get two slightly different tones which may fight each other both at the driver and at the port. By combining the airspace you are more likely to get a balanced sound (though this difference may be subtle). Another thing which you can do is operate your drivers in a semi-isobaric orientation, by having one firing into the enclosure, and the other firing out. Drivers will have different properties on the out stroke (away from the magnet) and their in stroke. This in fact is the entire point of a proper isobaric mounting. But this semi-isobaric setup would also help to compensate for this which will result is a small distortion drop.