Is Bluetooth and Wifi MAC address always sequential?
I imagine this is the case: iPhone and iPad are using an integrated chipset (like this: http://www.broadcom.com/products/Wireless-LAN/802.11-Wireless-LAN-Solutions/BCM4334) supporting both Wifi and Bluetooth. And as both of them are on the same chip the manufacturer could be just incrementing the MACs. 

However the question is why you ask this - if you want to use this information to check if it's the same device: don't, as there is no guarantee that it will always be this way.