Check out this new article Wizards of the Coast posted recently:

Ask Wizards: 03/20/2007

Q: Dear Sage,
When a dragon shaman (PHB II, 11) gains the ability to project an additional draconic aura from another source (such as the dragon lord prestige class; Dragon Magic, 38), would the new aura use the same bonus as his existing auras? Could he project an aura from each class simultaneously?
--Robin