You've never set one up, have you?
For instance, using a Netgear Prosafe firewall you can use 2 seperate lines. The best you can hope for is load balancing (eg: smtp on one, web on another) or failover. You cannot "shotgun" the 2 lines.
A sonicwall will give you similar functionality.
You cannot "bond" the 2 lines in the manner you are eluding to.