Loadbalancing 2 gateways

I have a windows XP pc with 2 IP addresses and 2 gateways (one in each range). i.e

IP address 1: 192.168.100.50
Gateway 1: 192.168.100.1

IP address 2: 192.168.50.50
Gateway 2: 192.168.50.1

The above is great for redundancy, i.e. if one gateway goes down XP automatically uses the other.

My question is … when both gateways are up, is it possible to loadbalance the 2 – either using native XP pro features or some third party app. i.e. one is a 1mb ADSL line and the other is a 2mb line, so I want a total of 3mb download from both gateways.

Hope this is clear.