Multilinking Modems

Broadband Internet access - Wikipedia, the free encyclopedia: "Roughly double the dial-up rate can be achieved with multilinking technology. What is required are two modems, two phone lines, two dial-up accounts, and ISP support for multilinking, or special software at the user end. This inverse multiplexing option was popular with some high-end users before ISDN, DSL and other technologies became available.
Diamond and other vendors had created dual phone line modems with bonding capability. The data rate of dual line modems is faster than 90 kbit/s. The Internet and phone charge will be twice the ordinary dial-up charge.

Load balancing takes two internet connections and feeds them into your network as one double data rate, more resilient internet connection. By choosing two independent internet providers the load balancing hardware will automatically use the line with least load which means should one line fail, the second one automatically takes up the slack."