Typically you don't unless they come from the same ISP. You might be able
to "load balance" them with a capable Router (must have at least three
interfaces). You bring both lines into the Router's 2nd and 3rd interface.
Then setup load balancing for those two links within the router. You
network then comes off the 1st Router interface which would go to your
Firewall or Proxy and then into your LAN or DMZ.
I personally don't think it can be done with two ISP, but that describes the
concept anyway. You need something that multiplexs the two links down into
one link before your private network can use it.
Another option would be to keep them separate and user two firewalls or two
proxys (one on each line) and then set half the user to use one and the
other half of the users to use the other.
Now if one line is going to go away because one ISP is replacing the
other,...then just forget the whole thing and wait till the final
change-over occurs. Or just switch to the new link and disconnect the old
one and leave it lay there until the phone company takes it out.
