Do you have ISP's that support that or do you mean something like "ISP Uplink/DSL Load Balancing" where the ASG utilizes two independent uplinks equally with fail-over?
I mean MultilinkPPP (MLPPP). My ISP supports single line (two PPOE sessions over the same physical link, usefull for bypassing Bell's throttling) or multiple lines bonded for additional throughput.