Actually, there are several ways to do it. There's one router from Nextland that's been mentioned on here several times, but it went away when Symantec bought them (so far as I know). I believe it was the PRO 800 Turbo - Might be able to find it used somewhere.
There are two other consumer dual WAN routers. The
Xincom Twin WAN router (which seems to have been designed by some guys from Hawking, after they were laid off during the aquisition) and a device made by Hawking - Their
Hawking Dual Wan Router. (Man, is it cheap! $55)
That being said..
Two cable modems on the same segment aren't going to buy you much, even when combined together. No matter how you bond them, you're never going to get a single download or upload to go any faster than it does with one - That's life with capped cable. What you could get would be TWO downloads or uploads going at max speed at the same time, one per connection.
The other caveat is that, when the modems are on the same wire, they compete for the bandwidth across the cable wire and across the rest of the ISP's infrastructure. Bandwidth, as a whole, is a finite resource. If it's busy for one cable modem, adding in a second one isn't going to buy you much.
In short... Don't mess with it, unless you really need it, there's little to be gained. The only real viable use for these types of setups is for redundancy - One cable modem and one DSL from another company. If one dies, the other survives. If you REALLY have to work at home and totally depend on your Internet to the tune of an extra $60/month, this is the best way to do it .
- G