It's been over 30 years, but my last stateside post was commanded by a major general. His second-in-command was a brigadier, and was referred to as the Deputy Commanding General (usually just as 'The DCG'). I assumed that above a certain rank, the number two was referred to as a deputy commander, rather than as an XO. Someone with more recent service than mine might be able to clarify this further.
Boomer