You can certainly get multiple DSL lines.. it's often done to get over the bandwidth limitations of the individual lines. As to cable modems - apparently
you can get multiple of those too. Maybe he had multiple addresses he was using though.
I mix them to maximize the odds I've got a working connection. I've got 1 cable & 1 dsl with load balancing and failover with a
SysWan router (highly recommended). Redundancy is better than if I were using 2 of the same - if for no other reason than the 2 lines go via separate physical paths out of the building. A side benefit is the load balancing makes things like downloads or youtube mostly non-issues.
In case you want to do redundancy too.. remember to look at everything. I've got redundant networks to my (redundant) computers - including one that bypasses the syswan in case it fails. Plus, my laptop is setup to use my mobile phone for internet access. Throw in a small UPS with major surge suppression for the cable & phone lines and I feel semi-safe for cheap. The last resort is to call my clearing firm on the landline, and have them place covering orders with floor brokers. I haven't had to do that for years, and I hope I never have to again.
The whole setup cost <$1k initially + <$100/mo. For a
residence in Chicago (at least), it can be done for half that. All of the hardware together - including two WiFi WAPs (on 2.4ghz and 5ghz) takes less space than an old desktop computer. Also the low power means no fans (so no moving parts to break, and no noise), and hours of run-time on a small UPS.
It's
insanely cheaper than what my old IT dept would've paid to do it, and it's not very hard to put together (networking is my weak suit in this business, fortunately colos have made that less of an issue). I don't know why more don't do it this way.
Oh, and one last thing - some people seem confused about standard DSL: Unless you've got an agreement otherwise, they're oversubscribed, just like cable. Yes - you'll get the full bandwidth to the other side of the DSL line, but that doesn't mean there's enough bandwidth for everyone to go full bore from there. Getting guaranteed bandwidth on DSL drives the cost up to where you should also look into T1s, fixed wireless, metro wans, etc.