I know a person that had 3 modems running, he had a "dual wan netgear" that the 2 modems were plugged into (so 2 x HFC mac addresses) how ever that was a dual load balanced router/switch, what I mean by that is it was dual load *upload* and fail over download, so if he was uploading a file then both modems would upload at the same time increasing upload speed, how ever for downloading then it used either modem 1 or modem 2, of course in the event of modem failure then the other would then kick in so he was never off line with that setup.
The other modem (so modem no.3) would run on it's own network (that was linked) and that used to supply his xbox 360 (so the other 2 modems could do all the downloading without it affecting the 360)
You could by a linksys and run some thing called "road runner" on dd-wrt custom firmware and that way you could configure one of the ports on the back to be a WAN and therefore run both modems at the same, how ever that was limited to 10/100 speeds so it would be pointless now as this was back in the day when 20 meg was the dogs.......
With the 50 meg and the 100 meg+ lines now you'd have to have 2 modems running on the line, so yours and a friends at the same time, could be done if some one was willing to loan you there super hub modem (which i doubt there would) and then what ever hard ware you bought you'd have to make sure that it was capable of "dual wan + bandwidth sharing at the same time) and you'll probably find that there quite expensive bits of kit. (once you start reading the nitty gritty about them then you normaly find that they do "fail over" and "load balance" means the upload rather than the download on the cheaper bottom of the market load balanced routers/switches.)
If some one was willing to loan you there superhub and you bought a nice piece of kit that shared the "downstream" and the "upstream" then you could do it.
I'm not quite sure how he managed to have 3 modems running at the time, something I never looked into.....