* Some ISP's offer this as a specific add on service. Sorry I do not have a list
* You can buy a load balancer that shares outbound connections across 2 or more adsl lines. You are still limited to the max speed of a single ADSL line, but 2 users could be using 1 line each
* you can use something line http://sharedband.com
which is ISP independent. That allows multiple ADSL lines to be added together and act line a single line.