Combining flat bar and aerobar: Double stem setup or alternatives?
A previous question regard rim and disc brakes had this link.... 

Essentially use a long steerer tube and mount two handle bars on the bike. Mount disc and rim brakes so you also have brakes on both handle bars (although you could one set of brakes off two levers.) 

Edit: (The original Link now broken so I have now linked to a Sheldon page (which still has the the broken link) in hope they restore it.) and attached the pic (although too small to be really useful)