Possibility (Score 4, Informative)

It seems possible to use a linux (or my preference -- OpenBSD) to NAT one or more internal addresses and round-robin each outgoing TCP connection from the NAT box -- alternate them between your two separate external addresses. Should be a pretty simply change to OpenBSD's pf NAT code -- if it isn't already there. Perhaps this could get you some higher incoming speeds? Maybe some coding here -- but it seems reasonable. Your internal machines should not care from which ADSL interface the traffic comes through since it's being NATed anyway. You just won't be able to load balance a single TCP connection.....