Are the ADSL lines into the same location? If your ISP supports multilink PPP over ADSL you could bond them, I believe RouterOS 9 will support this but there are other solutions (Cisco, Linux) already available.
I had same problem and have to use 3 computers. 2 computers are connected to ISP, and the third use those 2 computers as GW. So that is how to get 2 diferent gateways…