It will double his speed, assuming he's downloading two (or more) seperate files.
The cable modems themselves are capped, it's not the physical line to your house. So unless the cable company is oversold in that area, he'll be fine. DOCSIS spec basically allows for somewhere around 48mbit (too lazy to look it up) per segment for downstream. (a single modem in theory could do this, if it had the whole cable segment to itself)
Either buy that thinger, or do the exact same thing with a cheapo linux box on some old pentium hardware and 3 NIC's. Pay your local high school linux geek $100 to set it up, and provide autofailover.
If you have a dedicated box somewhere at a datacenter, and it's close by (latency wise) you could just tunnel both connections to it. Then you would be able to use the full speed of both for 1 download, (so you'd have 3mpbs downloads). Of course, the drawback to this is that you're using your dedicated boxes bandwidth too, and the added hops to get there.
Lots of options.
-Phil