Read through the manual real quick, and looks like its possible, which is rare fir a router. Its possible because it isn't controlling NAT sessions, its controlling the ATM session itself between the router and the modem. Handy.
You probably want to fallow page the instructions on page 26 to configure different ATM sessions for each interface. Hopefully your ISP allows this. Next, check page 69-71 of manual and see where to set the priority for those interface. There is alot going on here, and I am not familiar with this process. I would suggest contact Dlink, but you will be very lucky if you get anyone in normal technical support that would know about this. Wish I had better news for you. Its going to take some trial and error.