I've recently made a dual 12" sub box to suit my VN Calais... after looking at some other dual 12" sub boxes at jb hifi and autobarn, i realised the difference between mine and theirs was that inside the box the two sub are seperated from each other with a divider(at jb's and autobarn).. but mine isnt like this though, inside my box its just an open area, will this be a problem ... im just thinking in terms of the excursion of the subs, won't it be double becuse they are both in the same enclosure??

It will be fine, as long as the box was built and designed properly.

the only real benefit from having the box divided is that if one sub dies the other will effectively be working in an enclosure twice the size, but if both subs are working fine then there is no advantage, apart from extra bracing, but as SPL lancer said, if its built well, then no problems at all

And if one sub DOES die you can just use the old trick of temporarily reducing the internal volume of the box down to what's appropriate by using old magazines and/or books taped together (to the total volume you want to reduce the box by) and secure them inside the box. Then rip the motor assembly off the dead woofer and use the cone/basket assembly as a passive radiator until you get a new sub. hehe I've seen this done more than once.